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
Use include_context to share code between groups.

Rspec2 clearly has piss poor code re-use semantics. :-(
Localised spec specific setup code.
Tidied a couple of things up in the rspec2 upgrade.

- Added assert_xpath, assert_selector, etc. - they remove a healthy
  amount of duplication and make it easier to...
- Wrap all lines at 80 columns.
- Fixed nested have_tag style assertions - they worked with
  rspec_hpricot_matchers but the block is ignored by Webrat.
Only mix Webrat::Matchers in where required.
Added rspec upgrade to CHANGES.