Feature/channels

@asmeurer here are some batch channel manipulation functions.

```py
from binstar_client.utils import get_binstar
bs = get_binstar()
bs.add_channel('channel-name1', 'username' [, 'package' [, 'version' [, 'filename']]])
...
bs.remove_channel('channel-name2', 'username' [, 'package' [, 'version' [, 'filename']]])
```