Merge pull request #480 from atdt/master

Improve handling of preserved comments; make custom function helpers public
Update lessc.inc.php

use property_exist to avoid the inconsistent error message:
"Undefined property: stdClass::$parent"
when error reporting is set to ALL
Merge pull request #485 from mkdgs/patch-1

Update lessc.inc.php
Update lessc.inc.php

Fix: color: contrast(#000); generate error because light and dark was not optional, the contrast function not act like lesscss.org documentation.
also the thresold is fixed and not implemented like less.js

this change:
- make optional light color, dark color and threshold
- threshold act like less.js implementation
- add lib_luma (for threshold calculation)
Update lessc.inc.php
rebuild tests
implement data-uri([mimetype,] url) from less.js 1.4.0
data-uri: add tests and use lessc::findImport to locate the file
use the built-in php magic database instead of the system one
Merge pull request #500 from nyordanov/master

Implement data-uri([mimetype,] url) from less.js 1.4.0
docs typo
CS fixes
Travis improvements
Merge pull request #530 from GrahamCampbell/master

Minor Improvements
Set visibility of lessc::compileValue to 'public'

lessc::compileValue is a useful helper for implementing custom functions.