Add status return to can.send() and underlying platform_can_send() functions.

Now, can.send() returns a boolean result to indicate whether the message
was successfully sent or not. The various platform functions differ in how
they handled the situation where all CAN transmitters are occupied. In
particular, the STM32x and STR9 implementations ignored the status returns
from the underlying libraries and so if all the CAN transmitters were busy,
the latest message simply got thrown away. Now, can.send() returns true/false
to indicate if the message really did get queued for transmission. All
platforms should behave the same as before.
Merge pull request #36 from smartavionics/mb-can-send-result

Add status return to can.send() and underlying platform_can_send() functions.