Implements offline kernel caching

See kylelutz/compute#21

This adds program::build_with_source() function that both creates and
builds the program for the given context with supplied source and
compile options. In case BOOST_COMPUTE_USE_OFFLINE_CACHE macro is
defined, it also saves the compiled program binary for reuse in the
offline cache located in $HOME/.boost_compute folder on UNIX-like
systems and in %APPDATA%/boost_compute folder on Windows.

All internal uses of program::create_with_source() followed by
program::build() are replaced with program::build_with_source().
Test offline cache with Travis-CI