Add Gettext formatter to read and write to .po files

This uses the same style as android2po where msgctxt is used as the
key, msgid is the original string in the default language, and msgstr
is the translated version in the destination language.
Fix "require 'erb'" to work on case sensitive file systems

Tests were failing when running "rake" before this was done
Add tests for gettext conversion
Update documentation to reflect addition of gettext formatted files
Add escaping of strings that start with "@ " for android