adding test helpers for handling AR in a test suite

Use theses helpers in your test suite.  They all generally assume
Ardb has already been initialized by calling `Ardb.init`.

Closes #9.
cleanup to how the runner initializes Ardb

This removes a bit of duplications and allows for easier expansion
of init behavior.  No overall behavior change.
made MigrationHelpers a module_function

This allows for the same api when included on migrations while
making them easier to test b/c I can just call them as singleton
methods.
Merge pull request #27 from redding/kr-testhelpers

adding test helpers for handling AR in a test suite