Added clean_document and clean_document! methods that use Document parser instead of the default DocumentFragment

This allows you to validate the HTML string as if it were an entire document, not just a fragment.
Currently, the fragment parser strips html, head, body, etc. tags even if they are included in the whitelist.
Updated usage section of the readme with clean_document example
Bump version, update history.
Fix tests in Ruby 1.8.7.
Remove Gemfile.lock.
Add Ruby 2.0.0 to Travis. Remove 1.9.2.
Fixing two bugs found in the clean_document methods

1) Sanitize.clean_document was using the clean_document! method under the covers,
    and thus returning nil if no changes were made.
2) Sanitize.clean_document was not handling nil appropriately and would raise

Conflicts:
	test/test_sanitize.rb
Self-aggrandiving
remove nokogiri dependency for old versions

need to set ruby version >= 1.9.2 because nokogiri 1.6.0 requires this
Remove contributor email addresses.

They're just spam magnets.
Bump minimum Nokogiri version to 1.6.0.
Add 1.9.3 to Travis.
Release 2.0.4.
Add test dir to the gem. Fixes #76

Apparently this makes things easier for people who package Sanitize for
Fedora.
Merge remote-tracking branch 'upstream/master'