Expose bare minimum of a Path library/module

[kaste]

Seeing #3626 and #3631 it is really time to factor a Polymer.Path module to get these path manipulations right once and for all. Usually you set minutes to hours trying to understand something like 970d576

What we need:

• head(path)
  foo.bar => foo
• isDeep(path)
  foo.bar => true
  foo => false
• isAncestor(base, path)
  ('foo.bar', 'foo') => true
  ('foo.bar', 'bar') => false
• isDescendant(base, path)
  ('foo.bar', 'foo.bar.baz') => true
  ('foo.bar', 'foo.bar') => false
• translate(base, newBase, path)
  ('foo', 'baz', 'foo.bar') => 'baz.bar'
• matches(base, wildcard, path)
  ('foo.bar', false, 'foo.bar') => true
  ('foo.bar', true, 'foo.bar.baz') => true
  ('foo.bar', false, 'foo.bar.baz') => false

Note that these functions are pure and can be memoized.

[TimvdLippe]

Was fixed in #3633 but not properly closed.