Proposed change: Adding timestamping to received mssgs

[thomasonw]

Proposing to add additional value to CANFRAME structure: .time
(timestamp). Allows applications to apply aging decisions to received
messages.

[thomasonw]

This pull request also has a correction in it (Still working out details of how to use GitHub - sorry):

Corrected size of timer return values.

Correcting functions: get_internal_timer_value(); and
get_timestamp_value(); to indicate 16 bit returned values as opposed to
32 bit.

CAN_TIM & CAN_TIMESTP are 16 bit registers. Representing them as 32bit
could cause issues when one is attempting to do relative time
calculations and these 16 bit registers have been extended to 32 bits.