Start moving executables to pliny-*

Start moving executables that are very much specific to the framework
(i.e. it won't make sense for them to be overridden directly in the
project) out of `bin/` into the gem itself as a provide `pliny-*`
executable.

I think we'll probably want to do a similar think for everything that we
currently have in `lib/tasks`. If those operations stay inside the
template itself, it'll be more difficult for us to make changes to them
down the road. This might result in a list of executables like the
following:

    pliny-db-create
    pliny-db-drop
    pliny-db-migrate
    pliny-db-reset
    pliny-db-rollback
    pliny-db-schema-dump
    pliny-db-schema-load

This also gets us easy tab completion for free, and allows us to start
easily adding a variety of other administrative-type tools. Thoughts?