Datetoken

- Fixed `token_to_utc_date` not reading `token` keyword argument

- Refactored testing suite to replace `mock` with
`freezegun` as it is more convenient to work
with dates.
- Added features:
+ Support for setting time zones so as to
localize the `now`'s value prior to apply
token modifiers.
+ `token_to_utc_date` to evaluate a token and
get back the result in UTC
+ `eval_datetoken`: Evaluate a token and more
arguments to later return and `datetoken.objects:Token`
+ `datetoken.evaluator.Datetoken`: Helper to configure
tokens. Supports fluent programming
- Dropped features:
+ `datetoken.models:Token.from_string` in favor of
`datetoken.evaluator.eval_datetoken`

- Added features:
+ Chainable modifiers after snap modifiers
+ Partial Pratt parser
- Dropped features:
+ `complex_token_to_date` util
+ `simple_token_to_date` util
+ `SimpleToken` model
+ `ComplexToken` model

- Limit dateutil dependency versions

- Hello world!