[#208] Adds a contribution guide

The new git workflow is incluided in the Contribuiting file
Merge pull request #219 from eddie-ruva/chore/add_contributing_guide

[Issue #208] Adds a contribution guide
[#218] Fixes map interaction

Now marker placement on google maps does not update the address field
automatically. We update the marker position when a new address is
entered but not viceversa.
Merge pull request #220 from eddie-ruva/features/fixing_map

[#218] Fixes map interaction
Changes service requests placeholders.

Changes placeholders and text help in service requests form for a more
descriptive text.
Merge pull request #221 from eddie-ruva/chore/change-placeholder-service-requests

Changes service requests placeholders.
Authentication refactor and fix

- Facebook authentication now linking with user already registered via
  email.
- Asking for an email when the user registers via twitter.
- Refactor profile image fetch from oauth
Merge pull request #222 from eddie-ruva/fix/authentications_error

Authentication refactor and fix