Auto build of GTests and Travis building and executing it

Change how **GTests** are build. Now uses *ENV* variable to know were is GTest source/include/lib (**GTEST_ROOT** env. variable). Also, now CMake flag **BUILD_TESTS_Sigma** (-DBUILD_TESTS_Sigma) enables/disables building tests (by default is true), and Travis should now build it and execute it.
The side effect of this is that is necessary set this environment variable if someone desired to generate the unit tests. In Linux/Mac should be like (could be put in ~/.bashrc or similar file in Mac) :  

    $> GTEST_ROOT=/usr/src/gtest/
    $> export GTEST_ROOT

In Windows, In Control Panel / System there is place to set-up env. variables.

I apply this to Virtual Computer repository and is working very fine (See: https://travis-ci.org/trillek-team/trillek-tr3200-vm). The only thing that I don't know very well, is how will work in Windows. In the worst case, will advice that can't fin these environment variable, and will not generate the tests. 
Also, I renamed a bit some stuff in the main **CMakeLists.txt**, to clarify that the files in **/src/tests** (now /src/execs) are executables.

Finally, I must comment a few lines in  **/include/resources/Texture.h** as **GL_MIRROR_CLAMP_TO_EDGE** is not in OpenGL 3.2 core, and gives a compile error on Linux.