* Added boards constants for LED_BUILTIN and KEY_BUILTIN

Sources:
* esp320: http://wiki.sweetpeas.se/index.php?title=ESP320#Developing_using_the_Arduino_IDE
* esp32thing: https://cdn.sparkfun.com/assets/learn_tutorials/5/0/7/esp32-thing-schematic.pdf
* espea32: https://github.com/AprilBrother/ESPea32/blob/master/hardware/ESPea32-schematic.pdf
* hornbill32dev: https://github.com/ExploreEmbedded/Hornbill-ESP32-DEV/blob/master/Design%20Files/Schematic/Hornbill%20Devboard.pdf
* lolin32: values from existing header
* nano32: http://iot-bits.com/closer-look-esp32-nano32/
* node32s: https://github.com/lamloei/node32s-book/blob/master/ch01/node32s-schematic.pdf

* Replace BUILTIN_LED constant with #define

* Add build time partitions compilation

* Fix wrong definition of SS pin

* Add support for core debug level selection

* update idf libs

* Add hint

* Update README.md

* Update README.md

* Update README.md

* Added a basic WiFi UDP client example: a sketch that sends random bytes over udp.

* Updated udp example and included Python and Ruby UDP servers.

* Initial version of build script for @PlatformIO; CI integration for Linux, macOS and Windows

* Remove CI badges

* Add @PlatformIO to Travis.CI

* Initial version of build script for @PlatformIO; CI integration for Linux, macOS and Windows

* Remove CI badges

Add Travis build badge

fixes: https://github.com/espressif/arduino-esp32/issues/145

fixes: https://github.com/espressif/arduino-esp32/issues/146

example:
```cpp
//done once on WiFi init
configTime(-7200, 3600, "pool.ntp.org");
//get local time
struct tm timeinfo;
if(!getLocalTime(&timeinfo)){
    Serial.println("Failed to obtain time");
    return;
}
//print time
Serial.println(&timeinfo);
//print time with different format
Serial.println(&timeinfo, "%A, %B %d %Y %H:%M:%S");
```
fixes: https://github.com/espressif/arduino-esp32/issues/29

fixes: https://github.com/espressif/arduino-esp32/issues/143

fixes: https://github.com/espressif/arduino-esp32/issues/144

* Add files via upload

enable pulseIn()

* Remove optimistic_yield

* smartConfig support

* fixed bug, added example

* added _smartConfigDone = ture

* changed example name, added explanation

With this step we remove all dependencies, but git. Mac and Linux come
with Python by default, so they will keep using get.py to update the
toolchain and support binaries. Windows users have an "All point and
click" installation option that requires only git and Arduino to be
downloaded.
Fingers crossed :)

- get.exe is clickable and will soon download binary form of esptool,
which will really simplify installation on Windows
- get.py does not require any extra modules on Linux and Mac
- ```pip install requests``` required only on Windows (if not using
get.exe)
- Paths are made relative to the file in order to make the windows
executable clickable.

Change return value of UDP::endPacket() to match specifications

Ref: https://github.com/espressif/arduino-esp32/issues/138

Add ```pip install requests``` to install steps