more closely match what `asset_url` sass function should return in tests
(try to) simplify SassFunctions tests
add `spritely-background` sass function. Closes #2

combines `spritely-url` and `spritely-position` values
take underscored image names into account while simplifying options parsing

* takes advantage of `rpartition` to more efficiently (and clearly) split the options
* swaps where we `to_s` the hash keys in favor of symbolizing in fewer places
clarify some unique parts of the gem with comments [ci skip]
bump to 0.2.0