Added additional include path to search within, supporting Mac OS brew u...

This change resolves the issue where the include files for icu4c are not being found. They reside in the brew installed directory of `/usr/local/include/unicode`. The build was only searching in the `/opt/local/include` hierarchy common to MacPorts.

See these issues...
[https://github.com/node-xmpp/node-stringprep/issues/47](https://github.com/node-xmpp/node-stringprep/issues/47)
[https://github.com/node-xmpp/node-stringprep/issues/25](https://github.com/node-xmpp/node-stringprep/issues/25)

error experienced 
```
> node-stringprep@0.5.1 install ./node_modules/node-stringprep
> node-gyp rebuild

  CXX(target) Release/obj.target/node_stringprep/node-stringprep.o
../node-stringprep.cc:2:10: fatal error: 'unicode/unistr.h' file not found
#include <unicode/unistr.h>
         ^
1 error generated.
```