Add a "differential.createrawdiff" Conduit method

Summary:
"differential.creatediff" requires a mostly-parsed diff, but there's no reason we can't make DifferentialDiffs out of raw diffs.

This mainly serves to lower the adoption barrier if getting "arc" distributed is too much of a hassle.

Test Plan: Made a diff out of a raw block of diff text.

Reviewers: ffx, vrana, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2751