Remove Hash#extractable_options core extension
Remove Class core exension

Was not used anywhere
Remove Array core extension
Remove Nil core extension, and Object#try
Remove Time#strptime

Was feeling a little uncomfortable with this one. All the tests still
pass, though. So I checked ActiveSupport, and this method is not
overridden there, so IDK if it came from. But anyway, this is already
defined in the Ruby stdlib (at least for the current version). Since I'm
testing on 1.9.3, and it's passing, I'll assume works for all versions
specified in .travis.yml
Remove Hash#extractable_options?
Move Hash#symbolize_keys to Utility.symbolize_hash_keys
Remove unnecessary conditionals

Since we aren't monkey patching any more, there is no need to check if
these methods are already defined.
Rename Utility -> Helpers

Based on http://igor-alexandrov.github.io/blog/2013/05/23/attrio-typed-attributes-for-ruby-objects/#comment-914612622