Upgrade to RSpec 2.0

A first pass to get Nesta on RSpec 2.0. This commit

* Upgrades Rspec
* Moves spec.opts to .rspec
* Updates Rake tasks for Rspec
* Replaces defunct `have_tag` matchers with Webrat `have_selector` and `have_xpath`
* Implements the specs marked pending because Nokogiri :heart:s HTML5 more than Hpricot.

I haven't changed the syntax yet, but I'd recommend we update the specs for the new [`expect`-style][expect].

[expect]: https://github.com/rspec/rspec-expectations#expect-syntax