Improve documentation & delegate to API the grant_type management

• Add installation from "pip" in the README.md
• Add coveralls.io & Travis badges in the README.md
• Replace nose by pytest
• Delegate to API the grant_type management

Version 0.2.0

• Removed pycurl and shelve modules
• Use requests_toolbelt for video upload
• Simplified the access token storage, They are now stored in memory by default.
  But we recommand to store theme in a protected OS directory.
• Explained in the Readme how authentification work in the SDK, and how access tokens are stored.