Credential prompts

@stoodder @jeregrine 

Added prompts for credentials and help text so users can know what to expect. This mimics the functionality of the heroku toolbelt `heroku login --help`

What we see now is

```bash
covalence login --help

Usage: node covalence login [options]

Options:
   -u email@computer.org, --username email@computer.org   
   -p S0oP3rS3kr3T, --password S0oP3rS3kr3T               
   -t, --token                                            Your covalence API token (see your project dashboard)

 If no credentials are provided, you will be prompted for them.

Example:

 $ covalence login
 Enter your covalence credentials:
 Email: email@computer.org
 Password (typing will be hidden):
 Logged in as Test User <email@computer.org>
```