Fix NULLIF semantics

NULLIF now correctly returns a result with the type of the first argument. For instance:

    NULLIF(1.0, 2) => 1.0
    NULLIF(1, 2.0) => 1

Previously, the type of the result was the common supertype of both arguments
Add comment field in DESCRIBE TABLE
Update to new ColumnMetadata api
Change column comment to be empty string instead of null
Configurable Hive SerDe, InputFormat, OutputFormat in CreateTableAsSelect

Closes #1453
added number of runs and fixed time zone header
Factor a TestingPrestoClient out, add a single node server query runner.

This removes the query specific code from the DistributedQueryRunner
and adds a new QueryRunner that uses a real, albeit single-node,
embeded discovery TestingPrestoServer. Unlike the LocalQueryRunner
which fakes a presto engine, this is a full-fledged server.
Upgrade to Jax-rs 2.0 and Jersey 2.9.1
Change "verifier-test-" to "verifier-test:"
added readme for cassandra client socket options
Fix valueIsNull creation in Block.getRegion
Fix VariableWidthBlock.toString()
Print "SKIPPED" instead of an empty message when skipping a query
Fix NPE when control cluster is unreachable
Mark results as not matching, when number of columns doesn't match
Add exponential backoff when reading from S3

Closes #1332
Fix analyzer issue when "is not null" is used in join clause

Fixes #1479
Improve "too many failures" error message
Fix checkstyle violations
Fix zone offset minutes calculation

And a typo at return type of 'localtimestamp' in the datetime document

Closes #1482