Add OpenMVG_INCLUDES cmake variable

Now project using OpenMVG as library can just call
`include_directories(${OpenMVG_INCLUDES})` in its cmake configuration to
have all required headers.

All headers required by OpenMVG are included the OpenMVG_INCLUDES as
some of them does leak from OpenMVG headers to the user.
Remove FORCE from cmake variable modifications

This allows user to modify values of these variables, as force will
overwrite values set by user from cmake cache.
Add OpenMVG_LIBS cmake variable

Now a project using OpenMVG as library can just call
`target_link_libraries(target ${OpenMVG_LIBS})` in its cmake
configuration to have all required libraries.
Do not build examples and tests by default
Build tests and examples in travis-ci
Update BUILD with using as library documentation
Enable multithread build for Travis Continuous integration. #125
Update BUILD instructions and cmake build system. #124