add command line interface

This adds a command line interface so pycobertura can be used as a command line tool, not just as a library.

```
$ pycobertura --help
Usage: pycobertura [OPTIONS] COMMAND [ARGS]...

Options:
  --help  Show this message and exit.

Commands:
  diff  compare two coverage files
  show  show the coverage summary
```

```
$ pycobertura show tests/dummy.original.xml
Name           Stmts    Miss  Cover    Missing
-----------  -------  ------  -------  ---------
dummy/dummy        4       2  50.00%   2, 5
TOTAL              4       2  50.00%
```

```
$ pycobertura diff tests/dummy.original.xml tests/dummy.with-dummy2-full-cov.xml
Name          Stmts    Miss    Cover     Missing
------------  -------  ------  --------  ---------
dummy/dummy   -        -2      +50.00%   -2, -5
dummy/dummy2  +2       -       +100.00%
TOTAL         +2       -2      +50.00%
```