Better Documentation of MVEL Test

The mvel package is intended for tests of MVEL which are essentially tests for how one might use
MVEL with Moo, but not tests of Moo itself. I've updated the documentation of the package and the
test class contained therein to better reflect that.

This is cleanup from issue #14.
Testing Config Includes Factory

The extension mechanism for moo-mvel relies on moo-core picking up the new source property
factory that moo-mvel contributes. This test makes sure that a newly instantiated Configuration
object does indeed pick it up.

Related to #14.