Don't download files to master's working directory.

This should avoid exceptions caused by existing
files with different contents.

I also removed some unused code.
Add synchronization to Executor.updateDependencies() (SPARK-662)
Fix sys.path bug in PySpark SparkContext.addPyFile
Allow PySpark's SparkFiles to be used from driver

Fix minor documentation formatting issues.
Merge pull request #394 from JoshRosen/add_file_fix

Add SparkFiles.get() API to access files added through addFile().