Add library auto-detection when compiling a sketch

[ladislas]

This PR adds library auto-detection when compiling a sketch, which means that you don't need to list all the libraries used in your sketch.

For example:

If LibOne.h includes LibTwo.h and LibThree.h, you just need to:

#include "LibOne.h"

and you don't need to:

#include "LibOne.h"
#include "LibTwo.h"
#include "LibThree.h"

It also fixes speed issues for auto-detection introduced with #277 and referenced in #280.

I got it fixed by using := instead of = for USER_LIBS, SKETCH_LIBS and SKETCH_LIBS_DEPS.

It is open for testing and should be good for everyone. :)

[ladislas]

I just rebased the my branch on the current master with speed fixes and added back the automatic library detection. :)

Works like a charm :)

Looking forward to reading your feedbacks