Added documentation to timer and removed timer_setup()

timer_setup() should not be exposed in the header file, since we
do not want other functions other than timer functions calling it.
I also added documentation to the timer functions so that other
people can use it! I really should put these in a util folder for
future projects.
Added the interrupted error code

This error code is returned when a function was interrupted by
some sort of condition. The functions documentation will specify
what interrupts will cause this code.
Implemented amount in bartender_pour()

The amount recieved by the bartender is now poured in the glass.
Fixed bug in handler

We want the value from the command buffer not the response buffer.
Added startup code for bump sensor

We will be using an external pin interrupt since it is the best
way to do it unlike polling every step. More code to come.