Ruby 1.8 and Rubinius compatibility

I was able to discover some show stoppers on 1.8 and rbx thanks to the test suite.

This changeset fixes them:

* `String#force_encoding` not being present in 1.8 (http://stackoverflow.com/questions/4583924/string-force-encoding-in-ruby-1-8-7-or-rails-2-x)
* `Time` object instantiation (http://stackoverflow.com/questions/5071078/how-to-create-a-time-object-with-specific-time-in-ruby-1-8-7)
* `Dir.exists?` not being present in 1.8
* Different `Find.find` API between 1.8 and 1.9 (http://ruby-doc.org/stdlib-1.8.7/libdoc/find/rdoc/Find.html)