Link to icucore on Mac

Mac OS X already has an inbuilt, partial `libicu` implementation called `libicucore`, see e.g. [this guide](http://blog.lukhnos.org/post/6441462604/using-os-xs-built-in-icu-library-in-your-own-project). This pull request builds `node-stringprep` against the inbuilt `libicucore`, avoiding finicky Homebrew etc. installations of `libicu` (see #31, #37, #47).

The diffs feature:

* The entire `unicode` directory of `libicu` [version 49](http://site.icu-project.org/download/49) from the ICU project. This corresponds to the version on my OS X 10.8 machine.
* Changes to `binding.gyp` to build and link against `libicucore`.