(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.
Heh, yeah.  What I did is redundant, but I thought there was value in leaving the tests for both "false" and "some-string" just to illustrate the semantic point.