Make parser configurable.

Fixes #182.
Add spec coverage of parser configuration

Without this patch we don't have any spec coverage of the :parser
configuration addition in the previous patch.  This patch adds two tests
that makes sure the setup_puppet method configures Puppet with the
:parser setting.  We don't actually test to make sure the configuration
is read from Rspec, but the majority of the behavior in the method is
covered with respect to the parser setting.
Merge branch 'daenney-future-parser'

* daenney-future-parser:
  Add spec coverage of parser configuration
  Make parser configurable.