Formatter: JSLint XML

Created the JSLint XML formatter. Because JSLint XML has the `evidence` attribute which contains the erroring source code, I've added the source of the erroring node to the message objects created in `api.report` in `lib/eslint.js`.

Fixes #15
JSLint XML formatter now escapes special characters in the evidence and reason attributes.
Merge pull request #208 from iancmyers/formatter-jslint-xml

Formatter: JSLint XML (fixes #15)