Modify usage of throw in translations and add API wrappers.

* Wrapped throw(unset) in API function cuttlefish:unset/0.
* Added the throw({invalid, Reason}) protocol with wrapper function
  cuttlefish:invalid/1. This is used for translations that wish to
  convey that a configuration is invalid when discovered after the
  validation phase.
* Changed the throw(not_found) from cuttlefish:conf_get/2 into a tuple
  of {not_found, Variable}. This allows translations to call conf_get
  for settings that are expected to be present and a useful message
  about which setting was requested can be included in the error.
Merge pull request #100 from basho/feature/sdc/throw-api

Modify usage of throw in translations and add API wrappers.