Adds tests for column_args_to_indices.
Refactors and adds tests for cumsum.
Adds tests for sample_list_dict and sample_list_dict_low_mem.
Moves sample_list_dict and sample_list_dict_low_mem to convstructs.

This is a better location for these two functions, since they correspond
more to a particular type of data structure.
Refactors and adds tests for SortedTupleKeysDict.

Using the recommendation from StackOverflow (see
http://stackoverflow.com/questions/3387691/python-how-to-perfectly-override-a-dict)
I've changed the implementation of SortedTupleKeysDict to use
collections.MutableMapping instead of dict as its parent class.
Refactors TwoWaySetDict to subclass MutableMapping.
Rename make_csv_reader headers parameter to header.
Renames convutils to utils, convstructs to structs.

This serves two purposes:

1. It resolves any confusion between the package convutils and the
   module convutils.
2. It distinguishes a v2.0 installation from earlier releases, as a side
   effect.

Updates documentation.
Adds .travis.yml file for Travis-CI.
Updates copyright on license.
Adds Sphinx project documentation.