Adding text/regex-in?, link?, heading? predicates.

Added text-in? predicate.

Added regex-in? predicate.

Added link? predicate.

Added heading? predicate.

Updating readme.
Fixing filter in  predicate.
Optionally specify :text or :href for `link?` predicate
Using `some` instead of `seq` + `filter`.
Merge pull request #19 from tokenshift/master

Adding text/regex-in?, link?, heading? predicates.