Make input labels in Africa's Talking channel more specific

[pmallol]

Version dev-3d17906

1. Go to Channels
2. Click on 'Create new..' and select Africa's Talking
3. Go to https://github.com/instedd/verboice/wiki/ChannelHelpAfrica's-Talking to check for information

Obtained:
The Username input should be filled with the AfT Application username and the API Key is the AfT API key generated through their website.

As for now, the channel form does not specifies such requirements.
It would also help if the label for the Name input was renamed to "Africa's Talking Channel Name"