Tested with (all at the same time):
- MPU9250
- HTU21
- TMP102
- BMP180
- AM2320
- SSD1306

fixes: https://github.com/espressif/arduino-esp32/issues/76
fixes: https://github.com/espressif/arduino-esp32/issues/5
fixes: https://github.com/espressif/arduino-esp32/issues/91

make IPAddress.toString() const method, it doesn't modify anything
within the class.

Implementation for Timers 0-4 (no locks yet)

wait for data to be latched and increase timeout in attempt to fix clock stretch issues
Connected issues:
http://esp32.com/viewtopic.php?f=19&t=632&p=2832#p2801
https://github.com/espressif/arduino-esp32/issues/81
https://github.com/espressif/arduino-esp32/issues/53
https://github.com/espressif/arduino-esp32/issues/11

Add instructions for using as IDF component

Fixes issue where switch will not properly trigger if Arduino is used
in IDF as component.
See
https://github.com/espressif/arduino-esp32/issues/83#issuecomment-265326
071

- Link to Windows instructions
- Update instructions for Linux and Mac

Fix formatting and numbering

fixes: https://github.com/espressif/arduino-esp32/issues/76

need close(sockfd) before sockfd = -1, "_connected = false and close socket" means stop() function. (#73)

fixes: https://github.com/espressif/arduino-esp32/issues/72

currently ```esp_wifi_init``` have to be called in ```app_main``` or
WiFi will fail to boot. When possible to boot later, code will be moved
into ```_esp_wifi_start``` to be executed when necessary

use driver/gpio for now.

fixes: https://github.com/espressif/arduino-esp32/issues/66

fixes: https://github.com/espressif/arduino-esp32/issues/69