cr17_nodes mbed_interface

Node for interfacing with the mbed(s). In the architecture (at the time of v1.1), this is both the "Encoder processor" and "mbed_interface" nodes. If desired, they can be separate nodes, but they currently have one wiki entry because it has not yet been determined. Additionally, it will be fine to have a single node for interfacing with the mbed, and then a different node for publishing updates to the tf tree based on data from the interface node.

We should use the USB bus on the mbed, instead of the serial port. In Python, this should be doable through the USBHID library. Previously, we had difficulties using it with ROS (linking problem I believe) and ended up using a FIFO file for sending it from ROS.

Useful Links: https://developer.mbed.org/media/uploads/wim/hid_usb_intro_an249.pdf