Export shift
years now accepts any TimeType not just Date

Also migrated to using yearmonthday() accessor rather than extracting
atoms in code blocks
All functions accept TimeType rather than Date
tonthdayofweek created; fixed bugs

NEW:
- tonthdayofweek()

FIXED:
- default adjust() method signature
- added shifters test file to runtests.jl
- shift() test case expectations