Improve documentation for Crepe::Endpoint#head

The `head` method also takes an undocumented options hash that maps to
HTTP headers. Document this to make it more discoverable.

Signed-off-by: David Celis <me@davidcel.is>
Consistent documentation whitespace

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #41 from davidcelis/master

Improve documentation for Crepe::Endpoint#head
Update bad documentation

Rack Mount compiles expressions and constraints by path component, so
`^` and `$` should not be used.

Fixes #42.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>