Temporary workaround for issue #169

Need to reproduce this case in a unit test, either with the CLR putting
the type-cast in or with a manual type-cast, and then come up with a
more generic fix here.  Also should apply to the two-parameter lambda
converter.
Proposed correct fix and unit tests for issue #169
Merge pull request #170 from mfenniak/issue_169

Allow no-op casts in expression trees on parameters
Update rethinkdb_spec to upstream fb47c1461798e0a60b207d99f2f401ab4c40678a
Remove "base" from Reduce as this is no longer supported by RethinkDB
Compatibility for rethinkdb/rethinkdb#1382 changes

...AndReturnValues changed to ...AndReturnChanges, and return DML object
updated to support multiple updates in the operation via Changes array.
Stub out new Grouping & Aggregation methods, implement Group by Index

Introduces a new datum converter for grouped data,
DictionaryDatumConverterFactory, which only supports $reql_type$
GROUPED_DATA, not generic IDictionary support.  Some more of the old
grouping support is commented out with #if...#endif until all new
grouping support is completed.
Support Group() by 1, 2, or 3 functions returning keys
Max aggregate support for both grouped & ungrouped queries
Min aggregate support for both grouped & ungrouped queries
Avg aggregate support for both grouped & ungrouped queries
Sum aggregate support for both grouped & ungrouped queries
Make GroupingTests do setup in test fixture setup
Count aggregate support for both grouped & ungrouped queries

Note that the behaviour of Count when no records match the predicate in
the grouped query is surprising.  I've written to the rethinkdb
Google Group for confirmation that the behaviour is as-designed.
Contains aggregate support for both grouped & ungrouped queries
Cleanup commented out code from obsolete group-by support
Use IGroupingDictionary for $reql_type$=GROUPED_DATA

IGroupingDictionary is a type that's only supported for read, it's
automatically returned from Group() queries and grouped aggregations, and
it's also used by rethinkdb-net-newtonsoft because the GROUPED_DATA
reql_type has a special format that can't be intrinsically guessed by the
JSON parser.  IGroupingDictionary<,> is an IDictionary<,> so it shouldn't
even be noticed by the consumers.
Support IGroupingQuery.Ungroup()
Support .Map() and .Reduce() on IGroupingQuery

Allows user-defined aggregations using grouped-map-reduce, rather than
just the RethinkDB pretty supported aggregations.
Update release notes for RethinkDB 1.15 compatibility changes.