Fixes #17

Unlike other autonomous algorithm, loop must have access to
everything directly in the prepare method so it can properly setup
everything for validation.

This will allow to simplify checking for autonomous algorithm types

This is a specific type for algorithm using loop as they have
additional constraints with regards to classic autonomous
algorithms.

This allows for python2 and python3 support and is required
since we are currently providing a python2.7 environment.

Now both invalid case are treated as exception:
- Calling a loop using algorithm without loop_channel
- Calling a non loop using alggorithm with a loop_channel

The tuple should contain a:
- boolean for success or failure
- dictionary with the data to return to the algorithm

The format is:

"loop": {
    "input": {
        "type": "data_format_name"
    },
    "output": {
        "type": "data_format_name"
    }
}

This will call the validation method of the loop algorithm.

To distinguish from classic algorithms.

This allows to have a different process method signature
for algorithm using a loop block and no need to modify
existing algorithms.

New beat.cmdline tests don't need it anymore