(maint) Pin to rspec-core ~> 2.0

Without this patch the new rspec 3.0 or greater may be installed as a
dependency which poses a problem because rspec 3 is not fully backwards
compatible with rspec 2.  This patch addresses the problem by pinning to
rspec 2.0 but less than 3.0 as suggested to supress RSpec 3 warnings.
(maint) Update puppet version to ~> 3.0

Without this patch rspec-puppet spec tests currently run against Puppet
2.7 by default.  This is a problem because Puppet 2.7 is very old and no
longer maintained and changes to Puppet 3 are not being tested with
rspec-puppet.

This patch addresses the problem by updating the dependency on Puppet to
be the major version 3.
(maint) Add gem build and release rake tasks

Without this patch there are no tasks to build the rspec-puppet gem
package or release the package to rubygems.org.  This patch addresses
the problem by adding the bundler tasks to build and release a gem.
(maint) Bump version to 2.0.0 for release

This patch bumps the version of rspec-puppet to 2.0.0.  A new major
version is warranted because of the following changes noted by Joshua
Hoblitt.

There were several small bits of breakage and most (all?) of my modules
required forward porting work.

Some tests still had include_class, which I believe was long deprecated
anyways.

 * I ended up pinning the version of rspec-core as gem 'rspec-core', '~> 2.0'
   to silence rspec 3 warnings. It would be nice if a new release set this
   requirement it it's gemspec.

 * It looks like libdir handling changes broke rspec-puppet-augeas
   domcleal/rspec-puppet-augeas#15

 * All instances of verify_contents(subject, ...) broke as subject has been
   removed and replaced with catalogue. #235

 * The construct I had been using to check for exceptions in negative tests
   broke. expect { should }.to raise_error(...) had to be replaced with should
   raise_error(...)

The last 2-3 are definitely breaking changes from 1.0.0 and I believe this
warrants a major version bump. Otherwise, I expect 'rspec-puppet', '~> 1.0'
will still break a number of module's tests.