Fixed authentication using Bitbucket (and adding email retrieval)

Fixed authentication using Bitbucket as reported in #34, based on information provided at https://confluence.atlassian.com/display/BITBUCKET/OAuth+on+Bitbucket

Also added support for retrieving e-mail from Bitbucket based on https://confluence.atlassian.com/display/BITBUCKET/emails+Resource#emailsResource-GETalistofuser'semailaddresses
Actually that is enough..
```
email = ''.join([e['email'] for e in emails if e['primary']][0:1])
```
Even though you can have many `active` ones but only one `primary` it will be definitely `active`. Because from the UI you cannot set a `primary` that is not `active` (unconfirmed emails).