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