Add sphinx documentation

[urfeex]

We should add some sphinx documentation that we can include in our complete ROS documentation. That documentation should contain

• usage information
• Possible quirks
  • Timestamps of data
  • Frequency restrictions

[urfeex]

I see two tasks left to do:

• All pages should contain github source links
• The refs on the README don't render nicely on GH. It's probably easiest to remove the refs or replace them by links to the assembled documentation as soon it is merged there.