Fixes for 1.9.x

Hello. I've been updating a legacy Rails app to use Ruby 1.9, and ran into a stack overflow exception when trying to flatten an array that contained FeedNormalizer::Entry objects.

I discovered that Singular's method_missing recursed infinitely when passed a method whose plural form isn't implemented in the receiving object. In Ruby 1.8, the resulting SystemStackError was caught by the rescue statement modifier, so the super's method_missing was called and raised NoMethodError as expected (after a noticeable pause during the recursion). But in Ruby 1.9, SystemStackError is not a StandardError and thus is not caught by the rescue statement modifier.

This branch also contains a few other fixes to make all the tests pass on Ruby 1.8, and most of them pass on 1.9. There are still 4 failing tests on 1.9, but they are due to other 1.9 compatibility issues.