Fix inline functions for GCC on Mac OS X

The recursive inline functions in IoObject fail to build in GCC, while
they succeed in clang.

The inline directive was being set to "always_inline" for ObjC code
only, e.g. the ObjC bridge, so as a result GCC errors out when it
encounters these errors.

This commit introduces a new "IOINLINE_RECURSIVE" macro which uses
"always_inline" where possible, and falls back to "static inline"
otherwise.

Fixes #260.
Merge pull request #273 from mistydemeo/inline

Fix inline functions for GCC on Mac OS X