When writing a set of files, only write to valid directories.

This fixes issue #40
Modernize Gemfile source
Add encoding comment to ensure file is parsed as utf-8
Drop code that strips positionals from iOS strings
Merge remote-tracking branch 'GateGuru/bugfix/do-not-strip-positionals'
Fix incorrect handling of nested paths when generating string files
Clean up write_all_files.

We now ignore the current directory and its parent as neither of these
are meant to be checked in write_all_files. This could cause extra files
to be written unexpectedly.
Add Blake Watters to the list of contributors.