Adds #start and #mstart matchers

Hey,

This patch adds the `#start` and `#mstart` (multi-start) matchers.

 - `#start` generates `{ "$in" => [/^query/] }`

 - And given a string like `Pablo, Bruno, Fulano` then `#mstart` generates  `{ "$in" => [/^Pablo/, /^Bruno/, /^Fulano/] }` for [efficient mongo queries](http://docs.mongodb.org/manual/reference/operator/query/regex/#in-expressions).

Cheers :beers: