Support for Apache Spark

This patch adds basic support for launching a
Spark application on YARN in standalone mode.

- support for env specific config through client.cfg
- simple status tracking / error handling
- kills the yarn app on ctrl+c

Change-Id: Ia1b488eef74c2bade31c9c3fc38b8e508d6cee64
Move spark support to contrib

Change-Id: Idf9267c3467bf8c704ab94bc76066762c94dc35f
Add Spark example code

Change-Id: Iea8110bbe0a7bb77c89c733579290c45e386b982
Fix type

Change-Id: I8dfbdf0ab2b6f0a07a13ae169eba8d14553a4920
Add support for a MySQL target.

In this commit there's just the Target time. There's no CopyTask similar to the postgres implementation b/c it's still not needed. Not a big deal adding it later though, they are conceptually the same.
With the current code you could define at output() method which returns a MySqlTarget and on this output you can touch() to mark the task as completed. Luigi will play along and invoke the exists() method to check whether the task is completed or not - like any other target type
Update docstrings for luigi.Target

Also add it to the api reference.
Merge pull request #279 from jcrobak/target-docs

Update docstrings for luigi.Target
Merge pull request #274 from edwardnewett/master

Support for Apache Spark
Fix luigid when called with --background and without --logdir
add config value for default hiverc file
Merge pull request #280 from onlynone/master

Fix luigid when called with --background and without --logdir
Merge pull request #281 from p5k6/hiverc_config

add config value for default hiverc file
add another configuration path if a specific configuration file is set
Merge pull request #282 from MarkRoddy/master

Point to Configuration File via Environment Variable
Merge pull request #278 from rantav/mysql

Add support for a MySQL target.
added date parameter
simple test harness for period
actual period parameter