added travis building support

I've added initial support for building on travis covering the following rendering platforms:  
  1 - Dummy  
  2 - Export (latter)  
  3 - Ogre  
  4 - OpenGL  

This also uses GCC and Clang.

I've had to run cmake with the following arguments, otherwise it fails to build on the Ogre (3) rendering platform. 
-DMYGUI_BUILD_DEMOS=FALSE -DMYGUI_BUILD_TOOLS=FALSE

I've filed an issue for it here:
https://github.com/MyGUI/mygui/issues/24

Once solved, we can remove the above arguments for better coverage.