Upgrade path from References modules

[mikemccaffrey]

The hope is for this module to provide a general-purpose reference field that can better meet the needs of our users than the individual modules provided in References. Therefore, we should probably have an upgrade path so that sites that are currently using References can convert their existing fields to Reference fields.

@moonray Provided an upgrade path within the reference.install file in pull request #25, but for the time being, we can't require that all of the References fields be converted when a user installs Reference, so we need to find an alternate method for only converting the fields that are desired (possibly a reference_migrate submodule that can convert data from a variety of other field types).