Adds support for multiple strings files and utf-8 file storage.

This update adds two (hopefully) good features:

  1. Multiple .strings files  
    If you use multiple strings tables within your app like us, the `genstrings` command will create strings files for each one. To allow for this, we loop through each strings file in the language's directory and handle it on it's own.
  2. UTF-8 strings file format  
    Strings files as generated by `genstrings` are UTF-16 (with BOM). Unfortunately this doesn't play well with either mercurial or git. Both mark UTF-16 files as binary leading to no diffs unless you use an external tool that specifically does its own diffs for binary files. Converting to utf-8 fixes this issue. Xcode supports converting utf-8 strings files on build into the appropriate format for the target device.

Caveat: We don't use xibs, and as such I don't have an easy way to test xib support, so I've left that part of the script unchanged.