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/test_program.cpp

@@ -31,7 +31,9 @@ BOOST_AUTO_TEST_CASE(get_program_info)
     program.build();
 
     // check program info
+#ifndef BOOST_COMPUTE_USE_OFFLINE_CACHE
     BOOST_CHECK(program.source().empty() == false);
+#endif
     BOOST_CHECK(program.get_context() == context);
 }