Enable unit tests for `ArcanistJscsLinterTestCase`

Summary:
https://github.com/mdevils/node-jscs/issues/444 has been resolved in commit [[https://github.com/mdevils/node-jscs/commit/0de667bac2491ba04d930ff94e990965213ba36b | 0de667bac2491ba04d930ff94e990965213ba36b]], which means that `jscs` can be easily run on files with any extension.

NOTE: JSCS version 1.5.7 is required.

Test Plan: `arc unit`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9789
Refactor author handling in the Mercurial API

Summary: There were two callsites which needed some information from the username. Only one worked "correctly", causing `arc diff` to not amend commits anymore because the author could not be parser.

Test Plan: run `arc diff` with changes

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9797
Alphabetize `.arclint` linters

Summary: I'm a bit OCD with this, but I find the `.arclint` file easier to read if the keys are in alphabetical order.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9804
Move `ArcanistSpellingDefaultData` into a configurable JSON file

Summary: Currently, the `ArcanistSpellingLinter` loads data from `ArcanistSpellingDefaultData`, with no way to configure the linter from an `.arclint` file. Instead we should define a format for a "dictionary" file, of which the `ArvcanistSpellingLinter` can load and of which the paths are easily configured through `.arclint`.

Test Plan:
Updated the test case and ran `arc unit`.

NOTE: I have removed the `LINT_SPELLING_PICKY` and `LINT_SPELLING_IMPORTANT` constants and replaced them with `LINT_SPELLING_FULL` and `LINT_SPELLING_PARTIAL`. This was done because it simplifies the implementation considerably and makes customization of the `ArcanistSpellingLinter` simpler, but also because these constants were not widely used in the existing implementation.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9805
Patch: don't complain about reviewers not being specified.
This will break in the future when upstream arc incorporates T4631. Hopefully there will be a better solution by then