Element deletion (#2)

- Added capability to delete node in backend
- Added UI element and DELETE /node/:id endpoint for
  for user interaction. No need for delete mode anymore.
- A bit of refactoring in rsp.js to make it more readable.

Known issues: Deleting elements does not remove their
endpoints in GUI. As this is purely a display issue
ignore this for now. Tracked in #18
Remove "remove" mode button
Introduce nodeType string property for all FlowNodes.

- Preparation for REST compliant node creation
- Potential future use in GUI special casing for
  certain node types.
- Removes the danger of mistyping node types.
Give /node a REST compliant interface (part of #15)

- POST /node with a basic configuration of nodeType, x, y created a node.
  Returns the configuration of the new node as well as its location.
- GET /node/:id returns a nodes configuration
- PUT /node/:id updates a nodes configuration
- DELETE /node/:id deletes a node
Fix FlowNetworkSpec.scala unit tests.

- Fixed test relying on the old FlowNode interface.
- Played around with testing the Application object.
  Unfortunately it seems like there's some bad interaction
  between the actor system and the FakeApplication. For
  now the one test I wrote is commented out.
Get rid of explicit mode selection.

- Remove mode switch button group
- Make move button a part of nodeType group with empty type
  which has the same behavior as the old move mode.
- Switch back to move mode after add. This way you cannot
  confuse the modes anymore. Seems like boostrap makes switching
  this in JS unnecessarily complicated. You not only have to change
  the radio button selection but also manually toggle the active
  classes making buttons appear pressed.