Closes issue #1 : dpath does not respond properly to paths that begin with the separator character
Closes #3: Fixes all remaining hardcoded references to /, and adds new unit tests to ensure separator is honored
Closes #2: Adds a validator to dpath.path that can check by separator or complex regex, and raises dpath.exceptions.InvalidKeyName. The validator only makes sense for use in dpath.path.paths() currently.