Add failing test for imported mixin with wrong path

When relative URLs are on, and there's an image referenced via url(...),
if the URL is unquoted, the generated CSS duplicates the base path
Add visitor.js array-based function cache for faster lookups. perf / #1615
Optimize visitor.visitArray() for perf. #1615
Misc. perf optimizations. #1615
New input chunker in parser.js. #1615
Minor fixes. #1615
FIXES ISSUE #1453: let ModifyVar change numerical variables
Merge branch 'perf-tune' of https://github.com/fredburger/less.js

Conflicts:
	lib/less/to-css-visitor.js
This adds an additional `vars` parameter to the `parse` method. It allows a user to inject variables into a less string before compiling.
fix tests
some code review changes and test for the new extra/missing parens check
test parens false negative
jshint: eval option
jshint: boss
Experimental support for mixins with interpolated selectors.
 interpolated mixins, fixed: possible false match if variable contains `,`
Initial implementation of mixin guard `default` function (aka "mutually exclusive guards")
mixin guard `default`: `default` moved to functions.js + other minor improvements
Remove alpha from contrast calc, see #1724
Add link to docs project in contributing section