Warn when an @extend fails.

Still needs tests and !optional.
Use the correct extension in inline test files in extend_test.
Fix the formatting of the no-unification warning.
Add tests for extend warnings.
Add support for !optional.
Document @extend warnings.
Merge branch 'extend-warnings'

Conflicts:
	lib/sass/selector/simple_sequence.rb
	test/sass/conversion_test.rb
	test/sass/extend_test.rb

Closes #111
See #242
Fix @extend warnings on Ruby 1.9.x.
Add two more digits of decimal precision.

Closes #319.
Update vendor/listen.
Preserve single-line comments in multi-line comments.

Closes #101
Merge branch 'stable'

Conflicts:
	doc-src/SASS_CHANGELOG.md
	lib/sass/tree/visitors/to_css.rb
Clean up the description of the mix algorithm.
Merge branch 'stable'
Allow mixins and functions to be defined in nested contexts.

Closes #427
Fix conversion of @media with dynamic expressions.

Closes #419.
Disable less support for sass-convert.

Closes #122
Have EvaluationContext.options[:filename] give the current file.

Previously, it gave the top-level Sass file.

Closes #412
Mention !optional in @extend warnings.

Closes #461
Preserve newlines in nested selectors.

Closes #411