Add MultipleTsvFiles input source.

Allows you to specify an array of file paths that have the
correct Tsv schema. Includes a new test case for this source.
Project on fields before groupBy.
Really do the optimization.
Init with groupFields. Handle scanLeft and sortBy.
Don't use multiple parameter declarations

Pass in a Sequence of paths instead of varargs so that we can have
a single parameter list
Remove isBuferrUsed.
Merge pull request #475 from twitter/klin_speed_groupby

Project on input fields before groupBy.
Merge pull request #460 from rweald/add_multiple_tsv_input_path_source

Add MultipleTsvFiles input source.
Merge branch 'develop' of github.com:twitter/scalding into feature/optimizedmatrix