Added rising and lowering pin interrupts.

Two additional functions with little overhead. Added functionality for no
cost.
Fixed issue with Time handling unused tasks

UnusedEnd was set to point to the first pointer instead of the end,
meaning that as soon as a task is returned to the unused pile, all of the
following tasks are lost.
Standardized pwm response to invalid data.

Made it so all of the pwm based functions (motor/servo/pwm) respond to
invalid data safetly and consistently by not changing their current state.
Demo Cleaned Up

Pending testing
Changed demo pins to match booster pack
Merge branch 'master' of https://github.com/ut-ras/Rasware2013
Added Sonar code and crushed tabs
changed more pins and formatting
Merge branch 'master' of https://github.com/ut-ras/Rasware2013
added doxygen-style documentation to encoder.h and gpio.h
switched tenses, began documentation on pwm, motor, servo, and i2c
finished the rest of preliminary documentation, left TODOs in i2c and uart
removed unnecessary line in motor.h
deleting necessary comment in motor.h again...