add csvdiff utility using daff library

Adds a `csvdiff` command, similar to unix `diff` but for tabular data (see #12). Implemented as a simple wrapper around the [`daff`](https://github.com/paulfitz/daff) library. The diff is itself a csv, in [coopy format](http://dataprotocols.org/tabular-diff-format/), and has color highlighting when written to a terminal:
![csvdiff](https://cloud.githubusercontent.com/assets/118367/4837381/f0e972e8-5fd4-11e4-8821-aada07e3ce45.png)
The utility seems to work across the range of python versions that `csvkit` is supported for.