Require 'yaml' to fix broken test.

Fixes the only failing test when running on Ruby 1.8.7.
Fix and expand test_method_missing.

Singular's method_missing recurses infinitely and thus overflows the
stack when it is passed a method the including object doesn't respond
to.

On Ruby 1.8, the resulting SystemStackError is a StandardError, so it
is caught by the rescue statement modifier on line 14 of
lib/structures.rb. But in Ruby 1.9, SystemStackError is a plain 'ol
Exception and thus is not caught.

Prior to this commit, test_method_missing always passed before
method_missing was called because NoMethodError was raised during the
initialization of the Feed object (line 246 of lib/structures.rb) when
parser was called on the nil argument passed in. Fixed this by using a
valid feed object.

Also added an assertion to test the situation in which I discovered the
infinite recursion problem.

test_method_missing should now succeed on 1.8 and fail on 1.9.
Fix for many tests which fail on Ruby 1.9.x.

On Ruby 1.9.x, to_s defaults to inspect, which breaks the technique
used to parse the test data  filenames. Switch to File methods which
work on Ruby 1.8 and 1.9.
Fix infinite recursion problem on Ruby 1.9.

Check whether object responds to pluralized method before sending it to
avoid recursing into method_missing until the stack overflows. The
resulting SystemStackError was caught by the rescue statement modifier
in Ruby 1.8 (with a noticeable pause during the recursion), but isn't
in Ruby 1.9.

Also, remove an assertion that fails now that respond_to? is called.
Could find a way to fix it, but it's redundant.

This fixes test_method_missing on Ruby 1.9; however, there are still 4
unit tests failing on 1.9 for other reasons.
Merge pull request #7 from toddthomas/fixes-for-1.9.x