Add webmock and shoulda

[MaicolBen]

This PR includes:

• Added webmock to avoid call external APIs by default, an alternative could be mock lib/services mocking but it is easy to forget. Another alternative is vcr but it is more complex, webmock let you know the code to put in case you call the external API. If you want vcr, please make a comment here and we can discuss.
• Added shoulda as suggest in Add shoulda  #26. devise token auth seems to have an issue with shoulda where it doesn't recognize the uniqueness that devise add by default in the email, if you remove include DeviseTokenAuth::Concerns::User in user, the email validator isn't need to add.

Closes #26