Got rid of FFI function pointers by eliminating public static inline functions

The FFI no longer consists of function pointers, which makes it much easier to import them in other programs. Public functions that were declared static inline are now inline and exported in a seperate translation unit, as recommended by section 6.7.4 of the ISO C standard

Something similar was already noted in *include/chipmunk/chipmunk_ffi.h*:

    // TODO: get rid of the reliance on static inlines.
    // They make a mess for FFIs.

Legacy function pointers are still supported, though the use of
`-DCHIPMUNK_LEGACY_FFI`. An additional option in cmake makes it easy to
enable or disable this feature. Eventually, it can be removed completely.

There were also a few inlude-directives in `src/` that were inconsistent with some other files; I have changed them to what I thought they should be.

Tested on Ubuntu 14.04 with GCC 4.8.2.