Add untokenized SearchField

[ahoho]

This PR introduces a new StringField class to handle raw strings. Unlike CharField, StringField does not tokenize/analyze strings, but stores them in the index as-is. There are also faceted and multivalued versions of this field.

[denizdogan]

Would #1789 solve this too?

[acdha]

This definitely looks like a good candidate for rebasing against the current branch and shipping.