Make AccumulatorParam an abstract base class.
Remove use of abc.ABCMeta due to cloudpickle issue.

cloudpickle runs into issues while pickling subclasses of AccumulatorParam,
which may be related to this Python issue:

    http://bugs.python.org/issue7689

This seems hard to fix and the ABCMeta wasn't necessary, so I removed it.
Add LocalSparkContext to manage common sc variable.
Merge pull request #396 from JoshRosen/spark-653

Make PySpark AccumulatorParam an abstract base class
SPARK-541: Adding a warning for invalid Master URL

Right now Spark silently parses master URL's which do not match any
known regex as a Mesos URL. The Mesos error message when an invalid URL gets
passed is really confusing, so this warns the user when the implicit
conversion is happening.
Merge pull request #412 from pwendell/master-url-warning

SPARK-541: Adding a warning for invalid Master URL
Merge branch 'master' into localsparkcontext

Conflicts:
	core/src/test/scala/spark/FileServerSuite.scala
	core/src/test/scala/spark/RDDSuite.scala
Merge pull request #411 from stephenh/localsparkcontext

Add LocalSparkContext to manage common sc variable.
Merge branch 'master' of https://github.com/mesos/spark