Switch to JSon encoded POST requests

[benediktkr]

We should switch from using POST forms to json data. E.g. in POST /nodes/<name> there has been some type confusion because python-requests serializes a python boolean to "True" or "False", as strings - not booleans. Serializing with json works properly though.

To send json data with python-requests from the client:

>>> requests.post(API+"/resource", data=json.dumps({'bool': True, 'string': "Lokun"}))
<Response [200]>

And to read the json data in a POST request on the Bottle end:

>>> json_data = json.load(request.body)   # Not json.loads
{u'bool': True, u'string': "Lokun"}