SBP Protocol Implementation: Adding context ability for function pointer calls used my spb_process and sbp_sent_message. This enables C++ code to set an pointer to an object, which the passed function can use to access the context it executes in
SBP Protocol Implementation: Adding context ability for callbacks, Callbacks are no longer global but associated with a state, and updated tests
SBP Protocol Implementation: Get rid of old global callbacks linked list
SBP Procotol Implementation: Have sbp_state_init call the clear callbacks function
Updating docs
Fixed incorrect pointer type passed to clear callback state
Correctly test both IO context and callback context pointers
Merge pull request #13 from njoubert/master

SBP Updates from Niels

Pass-though context pointer to callbacks and read/write functions for easier C++ integration.

Many other small improvements.