Private access credentials

Implements bundler/bundler-features#3. Users can provide authorization credentials on a source-by-source basis by setting configuration options in `bundle config`:

```bash
bundle config https://gem.fury.io/smashwilson/ username:password
```

Usernames and passwords specified directly in the source URI still work (and will take precedence), so this is backwards-compatible. The error messages that are shown on authentication failure provide instructions for setting the credentials:

```
ash@autumn ~/temp $ dbundle install
Fetching source index from https://gem.fury.io/smashwilson/
Authentication is required for https://gem.fury.io/smashwilson/.
Please supply credentials for this source. You can do this by running:
 bundle config https://gem.fury.io/smashwilson/ username:password
```