remove xfail import
Move dispatcher objects to separate file
add extend method to Dispatcher

Serves as singledispatch style decorator
Rename Dispatcher.extend to register

This is similar to Py3.4 functools.singledispatch
add design doc
reword docs/design.md
reword docs/design.md
test MethodDispatcher.register
minor edits to docs/design
fix typos in design.md
add resolution doc
edits to resolution docs
Merge pull request #6 from mrocklin/refactor-1

Move dispatcher to separate file, add register decorator