support space-delimited rel-types and unquoted rel-types

Nice work!

I noticed that you are constructing new `RegExp` instances every iteration through your `for` loop. So I moved these out of the loop.

Also, I noticed that your expressions assume that the `rel=` value is going to be wrapped in double-quotes, however the W3C seems to think it's okay to not have the double-quotes for single relations:
http://www.w3.org/wiki/LinkHeader

Lastly, I noticed you don't support multiple relation-types in the one `rel=` parameter. The RFC indicates that these should be space-delimited:
http://tools.ietf.org/html/rfc5988
```
relation-types = relation-type
                 | <"> relation-type *( 1*SP relation-type ) <">
```
I didn't bother implementing Extension Relation Types (where the relation-type provided is a URL) as that's beyond my use cases.