Add Pry support for Pliny

As discussed in #98, let's pull Pry in instead of standard IRB. It's
very useful as a console in development and production, and also hugely
useful for debugging in development and testing. It also seems to be
fairly well standardized in that many projects want to see its
inclusion.

Fixes #98.
Pull in Shogun's prompt customization
Merge pull request #118 from interagent/brandur-pry

Add Pry support for Pliny