Implement Spark 1.6's new unhandledFilters API

[JoshRosen]

Spark 1.6 extends BaseRelation with a new API which allows data sources to tell Spark which filters they handle, allowing Spark to eliminate its own defensive filtering for filters that are handled by the data source (see apache/spark#9399 for more details).

This patch implements this new API in spark-redshift and adds tests.

[codecov-io]

Current coverage is 95.08%

Merging #128 into master will increase coverage by +0.01% as of c72ee35

@@            master    #128   diff @@
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  Files           12      12       
  Stmts          508     509     +1
  Branches       123     123       
  Methods          0       0       
======================================
+ Hit            483     484     +1
  Partial          0       0       
  Missed          25      25       

Review entire Coverage Diff as of c72ee35

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[rxin]

LGTM