Optimized Selector Variant

[josharnold52]

As of #51, custom selectors can no longer leverage the bloom filter. I'd like to try and restore that ability in a more (hopefully) palatable way:

trait OptimizingSelector[+A] extends Selector[A] {
  /***
   * Provides a hint as to whether the selector can any children or their
   * descendants in the specified group.  Note that this is merely a hint,
   * `isDefinedAt` is the authoritative determinant of whether a selector matches.
   */
  def canMatchIn(group: Group[Node]): Boolean
} 

Basically, canMatchIn takes the role of the old matches method, but with (hopefully) cleaner semantics. Of course, implementations can call Group.matches(String) to reproduce the old functionality.

For some concrete examples, here are some searches that I'd like to be able to do while leveraging the bloom filter optimization:

1. Find all elements, e that have a given name and a given attribute value
2. Find all elements, e with a given name and return f(e) for some transform f.
3. Find all elements, e that contain a child element with a given name.

[djspiewak]

Ok, yeah, that is a lot more general than just matching on ElemSelector…and you provided tests! Merged!