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/
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
[hadoop scheduler] switching between schedulers using hadoop_scheduler config parameter
Support optional serialize() method on Parameter
fix LocalFileSystem.isdir
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
Hadoop binary path can now be specified in config.  Sets are converted to tuples in Task class.