get_channel_id function added

* get_channel_id function return the channel id from channel name.
* mocking test added.
* requeriments-dev.txt file added.

Some slack API methods need the channel id, not the channel name, like `files.upload`.

Channel id ('C02GXXYZ') is different to the channel name ('test').

Example:

```
>>> from slacker import Slacker
>>> from slacker.utils import get_channel_id
>>> token = "aaa"
>>> channel_id = get_channel_id(token, 'test')
>>> slack = Slacker(token)
>>> slack.files.upload('index.png', channels=channel_id)
```