Code reorg

When including this gem via bundler, using the following directive in the Gemfile:
gem 'plist_read_write', :git => "git://github.com/KatanaCode/PListReadWrite"

...and adding the gem to the project Rakefile, I was getting the following error: 
"The plist_read_write gem must be required within a RubyMotion project Rakefile."

I determined that if you relocate the module code to a subfolder, and leave the build-related code at the top level, this error wouldn't occur. This pull request has that change applied.