Support unixodbc in Mac OSX

This resolved the MemoryError issues that I had experienced with iODBC. [It seems that others have this issue as well.](https://code.google.com/p/pyodbc/issues/detail?id=307).

Btw what is the status of the pyodbc project? There's [a lot of unanswered issues](https://code.google.com/p/pyodbc/issues/) these days.