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
```
email = ''.join([e['email'] for e in emails if e['active']][0:1])
```

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