Allow for ignored components

Hi, this is a patch that allows to strip out specific uri components passed to __call__().
This is util where non-validated data is used to build some url but it is ok if missing.

Silly example:
<pre>
 >>> h=hammock.Hammock('http://url.com', ignore=(False,None))
 >>> h.goodbye(True, False).cruel(None).world.False
 http://url.com/goodbye/True/cruel/world/False
</pre>