Task history, implementation of https://github.com/spotify/luigi/issues/51

All the tables are added as per the RFC, though currently
only Task, TaskEvent, and TaskParameters is being used right now.

Task history is added by the scheduler, history viewer
available through /history/
[job conf] add support for job queue parameter (for Capacity Scheduler)
Merge pull request #120 from kawaa/master

[job conf] add support for job queue parameter (Capacity Scheduler)
Updating code to run in both CHD3/4 and Hortonworks current releases

hadoop fs -ls -d is not compatible with releases other than CDH4. This update should work in all versions.
Updating code to run in both CHD3/4 and Hortonworks current releases

Updated code after the review
Merge branch 'master' of git://github.com/kawaa/luigi into kawaa-master

Conflicts:
	luigi/hadoop.py
	luigi/hive.py
Merge pull request #127 from erikbern/queue-pool

[hadoop scheduler] switching between schedulers using hadoop_scheduler config parameter Edit
Support optional serialize() method on Parameter
Merge pull request #129 from erikbern/serialize-params

Support optional serialize() method on Parameter
fix LocalFileSystem.isdir
Merge branch 'task_history' of git://github.com/JoeEnnever/luigi into JoeEnnever-task_history

Conflicts:
	test/customized_run_test.py
Merge pull request #132 from erikbern/task-history

Task history
Merge pull request #131 from spotify/neville/isdir

fix LocalFileSystem.isdir
added Parameter class to accept float values
Merge pull request #133 from miku/minor/floatparameter

added Parameter class to accept float values
support dependency introduction through inheritance

Facilitates task composition through inheritance, as I desired for e.g. a generic Hive partition creation task, which would besides input require also table creation.
Cleanup in the RPC mechanism for forwards compatibility + a bunch more unit tests
Merge pull request #121 from vivshri/master

Fixing file existence check to make it compatible with various hadoop releases and distributions
Merge pull request #135 from ulzha/master

support dependency introduction through inheritance
Merge pull request #136 from erikbern/rpc-cleanup

Cleanup in the RPC mechanism for forwards compatibility + a bunch more u...