[C154_I] Added Alphabet class.
[C154_I] Final changes to C154_I.

- Added Letters property to Alphabet
- Alphabet.InvalidationMessage returns a list of messages
- Removed extraneous logic from Sorter class
- Renamed Sorter.SortedWords to Sorter.Sort
- Re-factored Sorter to take no construction arguments, but instead take
  arguments for Sort method
- Re-factored Parser to not collect words if the alphabet is invalid
- Re-factored Processor to print sorted words when valid and error messages
  when invalid