OneDrive backend - give proper error message if requests or requests-oauthlib is not installed

I was trying to use the OneDrive backend for backups and kept getting the following error message:
  UnsupportedBackendScheme: scheme not supported in url: onedrive://backups

I took some time to track down where the issue was and it appears that if you happen to run duplicity with --verbosity info a following message would appear:
  Import of duplicity.backends.onedrivebackend Failed: No module named requests_oauthlib

So, I had not installed python-requests-oauth and the backend failed to load but the error message I was seeing (without --verbosity info) give no clue about what was going on.

The OneDrive backend could do what multiple other backends do for dependencies that might not be installed by default. That is import requests and requests-oauth inside the backend initializer, catch any errors and give a clear instructions what to do when that happens.

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Duplicity version: 0.7.02-0ubuntu1
Python version: 2.7.6
OS distro and version: Ubuntu 14.04