Change find_by_* to return nil rather than raise an exception.

This is a breaking API change.

You probably have code that looks like
  if emoji = Emoji.find_by_alias($1) { nil }
that you can change to
  if emoji = Emoji.find_by_alias($1)
Merge pull request #54 from jeremy/nil_style_find_api

Change find_by_* to return nil rather than raise an exception