Begin writing support for DateTime constructors

The simple template mapping approach here gets it slightly right, but
it needs to support DateTimeKind in the constructor to begin to get
DateTimeOffset correct.
Work-in-progress on DateTimeOffset constructors

A complicated constructor to support new DateTimeOffset(dt, offset) that
converts TimeSpans to offsets, server-side, by formatting the correct
offset string.

It seems like the DateTimeOffsetDatumConverter isn't handling the offsets
the way I would expect; I've made a change here, but it breaks the tests.
The tests need to be re-evaluated to ensure my change is accurate...
it might not be.
Improved tests around DateTimeOffsetDatumConverter

Took raw values out of RethinkDB for time constructions in local and with
offsets, and ensured that DateTimeOffsetDatumConverter matches both
creating the objects and the datums for both cases.  Confirmed that
offsets were not working correctly before these upgrades.
Fixup newtonsoft tests to correct DateTimeOffset serialization
Unit tests for all DateTimeOffset constructors w/ server-side inputs
Update release notes
Merge pull request #184 from mfenniak/issue_154

Support for DateTime constructors in expressions; fixes #154.