Allow pre existing connections to be used

[nealio82]

Depends on #19

This PR breaks the config into encapsulated classes and removes a lot of the coupling between the migrator and the App class. It allows injection of connections into the migrator, so pre-defined Connection objects can be used, rather than requiring using the config.json file. It is a first-step in tidying and abstracting some of the parts of the system, with the goal of moving toward SRP and cleaner APIs between objects.

I've also added unit tests for some of the classes I've introduced, while being careful not to modify the existing integration test suite.

[michaeljoseph]

Hey @nealio82, could you please bring this up-to-date with master now that we've fixed the build?

[nealio82]

hey @michaeljoseph, i rebased onto the updated master

[nealio82]

@noondaysun normally I'd have batch-applied your suggestions so you get onto the authors list as well, but the change in Migrator.php needed to be done in the IDE as I needed to delete a few lines lower down too, so I did the whole lot in one go - but thanks for jumping in!