(PDB-158) Added option to disable update checking.

- Updated documentation and sample config file.
- Added `disable-update-checking` configuration parameter with a default
  value of `false`.  If set to `true`, then PuppetDB will not spawn a
  future thread to check whether an updated version is available.
- Removed :updater from post-condition check, in case it's nil.
- Added check to skip adding nil :updater to context.
- Added new parameter to puppetdb-config-in schema, created -out schema, and
  converted input to output configuration.
- Added test code to check for proper config conversion, expected exceptions,
  and correct default value.
It'd idiomatic here to put expected on it's own line (or better yet, just put literal map directly in the = below). Also idiomatic to use a vector [:disable-update-checking] rather than a set below.