Fixup pull request
Simplify tests
DurationField is incorrect in complex DateTimeField instances

Fixes #92
Fix NPE in DateTimeUtils.isContiguous with weird partials [#89]

Fixes #89
Prevent creation of invalid partials via Partial.with(DateTimeFieldType,int)

Validation in this method was not as thorough as that in the constructor
This allowed invalid partials to be created
Fixes #88
Adding zero no longer changes the offset during DST overlap

Fixes #77