Prefer variables over properties of `jade` global

Switch `jade.interp` with `jade_interp` and `jade.indent` with
`jade_indent`.
Merge pull request #1398 from visionmedia/less-jade-global

Prefer variables over properties of `jade` global
Add test case for lazilly adding mixins
Support lazilly adding mixins

[closes #265]
Added the MIT license to the package.json.
Merge pull request #1407 from keskival/master

Added the MIT license to the package.json.
Add test case for fall-through

[see #1363]
Fix case fall-through

[closes #1363]
Merge pull request #1406 from visionmedia/lazy-mixins

Lazily add mixins
Merge pull request #1408 from visionmedia/case/fall-through

Fix case fall-through
Require error for `when` without `case`
Require error for `else` without `if`
Ensure error messages contain filename and line no
Test support for empty blocks in conditions
Implement error handling and empty blocks
Error for a tag other than `when` or `default` inside `case`
Merge pull request #1412 from visionmedia/error-handling

Error handling
- Jade prototype: async rendering with co and yielding locals