Adding header for OS X compilation

Add OS X support

To allow native extensions to compile on OS X, I found it was necessary to copy the mecab.h file from the Mecab binary source code.

CentOS (our other deployment target) continues to build the Gem with no issues as far as I can see.

If there is a better approach that allows this Gem to be used in Gemfiles that are deployed on both CentOS and Mac OS X, would be happy to see that.