[spin1_api] Also use timer1 to get the number of microseconds elapsed since the last tick
[spin1_api] Added utilities and isr using timer2 to schedule a callback to be triggered with a certain microsecond delay
[app_frame] Added two new models for microsecond precision experimentation

 * DendriticSyncrhony : detects the synchronous arrival of two spikes at a given neuron
                        (simulating dendritic summation of action potentials)
 * DelayUs : emulates a dendrite compartment by delaying a spike of a certain number of microseconds