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
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...
allow running without Tornado
Merge pull request #139 from ulzha/master

allow running without Tornado
Add-fix tornado bug

Only load tornado dependent classes when tornado is available.
Merge pull request #140 from wouterdebie/master

Add-fix tornado bug
Rewrote README.md and added the examples from the PyData 2013 preso by Elias

RFC: let me know if this makes sense.

Updated pictures to use the new visualizer, except the big splashy dep graph which I'll do later.

Removed some crap in examples/ and doc/ and added some more random stuff to README.md
Merge pull request #143 from erikbern/new-doc

Rewrote README.md and added the examples from the PyData 2013 preso
Minor fixes in readme
Minor fix about triggering many tasks
Fixes visualizing a task graph with missing tasks

They show up as UNKNOWN (like luigi-grep). Prints out a warning message
when this occurs, though it can be quite noisy.

Fixes https://github.com/spotify/luigi/issues/122
Merge pull request #144 from JoeEnnever/vizfix

Fixes visualizing a task graph with missing tasks
Refactored the task history support added by jennever@foursquare.com