script/lexer: properly calculate source_range in `scan`

Fix off-by-one error when calculating offsets in `scan`.
Fix a documentation bug.

Closes #1234
Properly set default options in Rails 3.0.

Closes #1235
Add a test for source range calculation and fix a related failure.
Add a changelog entry.
Merge branch 'source-range-calc' into stable

Conflicts:
	doc-src/SASS_CHANGELOG.md

Closes #1236