Providing testing capabilities to outside packages

[tobithiel]

The testing methods used inside spark-redshift are unfortunately all marked private and can't be used from the outside. We were looking for a way to run our tests against local mocks instead of the real online services. We got it working but had to do few minor adjustments to spark-redshift. Let me know if we could integrate these into the official version.
Our changes are:

1. We need an ability to provide a custom S3 client factory to spark-redshift, so we added an extra constructor to DefaultSource. The current constructor also requires a JDBCWrapper, which is marked private and therefore doesn't seem to be able to be passed in from the outside.
2. We're redirecting the Redshift queries to a local PostgreSQL instance (with a small compatiblity layer redshift-fake-driver). PostgreSQL unfortunately requires all subqueries to have an alias, which we added to the unload query generation.

Let me know what you think and if this could be merged in.