SPARK-1111: URL Validation Throws Error for HDFS URL's

Fixes an error where HDFS URL's cause an exception. Should be merged into master and 0.9.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #625 from pwendell/url-validation and squashes the following commits:

d14bfe3 [Patrick Wendell] SPARK-1111: URL Validation Throws Error for HDFS URL's

(cherry picked from commit 45b15e27a84527abeaa8588b0eb1ade7e831e6ef)
Signed-off-by: Aaron Davidson <aaron@databricks.com>