Merge pull request #1 from blaugueux/master

Added new method to resubscribe an existing contact.
Update api doc URL.
Refactored SDK with Guzzle.
Merge pull request #3 from blaugueux/2.0

[WIP] Complete refactoring using Guzzle.
Prepare 2.0.0 release
Bump version to 2.0.1-dev
Add new account properties.
Prepare 2.0.1 release
Bump version to 2.0.2-dev