Pass importer to the script parser
Add high-level selector source range support for sourcemaps
Merge pull request #638 from crdev/script_importer

Pass importer to the script parser
Address reviewer comments
Merge pull request #639 from crdev/selector_ranges

Implement selector source range tracking
String function for length
String function for inserting a string into another.
String function for finding the index of a substring.
String functions for case manipulation.
String function to extract a substring
Remove unnecessary module scopes in the function documentation.
Documentation improvements for the string functions.
Make it easier to construct a Sass::Script::Number in the common case for hand-coded objects.
Add a method to Sass::Script::Number to make it easier to check the unit of a number.
Add assert_unit helper function for making sure the correct numeric unit is used when calling a ruby function.
Doc improvements for the str-index function
Rename str-extract to str-slice, improve the documentation.
Assert that arguments to ruby functions that must be a integer are.
Changelog entry for new string functions. Closes #401.