Add config key to customize default 'Read More'

The user is provided the config key `read_more` which will set the
default text used for 'Read more' links throughout the site. If the
`read_more` config value is not set, the site will use the previous
default of 'Continue reading'. Users can still customize the 'Read more'
text on a per-page basis with metadata.
read_more doesn't have automated behaviour; has its own method.
Whitespace fixes.
Updated CHANGES for read_more config.
Add hooks for menu and breadcrumb classes

Plugins or site authors may want to customize the css classes used for
the current menu item and the current breadcrumb. This commit adds those
hooks.
Coding style.
Updated CHANGES for menu/breadcrumb styling.
CLI completion for zsh
Sort options alphabetically.
Mention --zsh-completion in CHANGES.
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.
Ensure page served when asserting tag not present.
Removed heroku gem from template Gemfile.

It hasn't been used since commit bc9abe removed support for it anyway.
Remove shotgun dependency

Fixes gma/nesta#125
Only install shotgun in development.