Remove dependancy on glide and instead switch completely over to using
`go mod vendor` as our vendor solution.

The repository is being switched from the personal account
github.com/derekparker/delve to the organization account
github.com/go-delve/delve. This patch updates imports and docs, while
preserving things which should not be changed such as my name in the
CHANGELOG and in TODO comments.

Fixes #1378

Support for position independent executables (PIE) on the native linux
backend, the gdbserver backend on linux and the core backend.
Also implemented in the windows native backend, but it can't be tested
because go doesn't support PIE on windows yet.

On macOS 10.14 Apple changed the command line tools so that system
headers now need to be manually installed.

Instead of adding one extra install step to the install procedure add a
build tag to allow compilation of delve without the native backend on
macOS. By default (i.e. when using `go get`) this is how delve will be
compiled on macOS, the make script is changed to enable compiling the
native backend if the required dependencies have been installed.

Insure that both configuration still build correctly on Travis CI and
change the documentation to describe how to compile the native backend
and that it isn't normally needed.

Fixes #1359

Correctly locates the installed executable path for darwin

We were using our makefile not for the intended purpose of makefiles
but to multiplex a series of small scripts.
We can easily achieve the same result in a more reasonable programming
language and as a bonus we don't need to install a version of make on
windows anymore.
Additionally our test script had become fairly complicated and will
become even more complicated when testing of PIE buildmode is
introduced.
Allows the tests to run on Windows without having to install Mingw
(although we still want it installed so that we can run cgo tests on
Windows).

Fixes building when GOPATH isn't set.

Fixes #759