added 'rebeccapurple' to named colours http://dev.w3.org/csswg/css-color/#valuedef-rebeccapurple
Properly check nesting within @at-root.

Closes #1294
Merge branch 'stable'

Conflicts:
	test/sass/scss/scss_test.rb
Merge remote-tracking branch 'pdaoust/stable' into rebeccapurple
Add a CHANGELOG entry for rebeccapurple.
Merge branch 'rebeccapurple'