Add YAML validation

[matthewjwhite]

Add YAML validation

[matthewjwhite]

See this PR's description for remaining stuff to ensure total validation: #21.

[matthewjwhite]

Added Yamale to handle structural validation - implying content as well by this, to the point that we're verifying basic attributes such as length, etc. Filled in the gaps that Yamale doesn't handle using extra logic in the Config constructor.

[matthewjwhite]

I wonder if DB documents should also be validated when loading them into memory as objects, since **kwargs craziness is used to load them as well... Probably not a huge necessity since nothing touches the DB but this. Perhaps the easiest/most valuable thing at this point would be to confirm that the DB user's class and location map still exist in the configuration.