libvw_c_wrapper.so and example how to use it from C program

Working example of using libvw from C program on linux (compiled with gcc).

It is based on C# wrapper (vwdll.cpp/vwdll.h) but modified a little to compile with gcc.

My work was based on previous discussion in mailing list:
https://groups.yahoo.com/neo/groups/vowpal_wabbit/conversations/messages/2256

(this pull request is a cleaned up version of https://github.com/JohnLangford/vowpal_wabbit/pull/387)