Persistance

Persistence has been implemented for the gateway, renter, and host.

The daemon now makes use of a general siaDir, where (as per #387 (closed)) each module stores its configuration data in a subfolder.

This works fairly well, except for testing. As it stands, the tests are using "" as their module subfolder, which results in a lot of .dat files littering the repo. I didn't bother to add all of these to the .gitignore and make clean; I think we should instead focus on cleaning up our testing framework so that all of these files have a sane place to live.