Change the install name of datatable.so on OS X

The build changes data.table.so to datatable.so, but it doesn't change the install name on OS X (`otool -L datatable.so`), so future things that link against it will pull in the wrong install name. The build should use `install_name_tool -id datatable.so path/to/datatable.so` to change the install name to match the file name (or probably better, compile it as datatable.so in the first place). 
