Avoid using event callbacks for mono compatibility

[glopesdev]

This PR refactors the serial interface to use an interruptible polling strategy to allow the Pulse Pal package to run cross-platform using the Mono framework.

Fixes #11

[RoboDoig]

This doesn't necessarily need to be resolved for this PR, but I notice now that the behavior of the ProcessInput method needs to be slightly different depending on what command caused bytes to be read in the serial reading task.

Most commands return a single byte, but the value can represent a voltage, state etc. depending on what the initiating command was. Moreover, some commands will return sequences of bytes, for example the handshake command that is used to determine firmware version returns the ack character followed by 4 bytes, in this case ProcessInput needs to operate on a byte array rather than a single byte.

One option might be to write an op code into the readBuffer on every command. ProcessInput is always called on the full readBuffer rather than a single byte, and can expect that the first byte will be a context op code determining how it should treat the remaining values in the readBuffer.

[glopesdev]

This doesn't necessarily need to be resolved for this PR, but I notice now that the behavior of the ProcessInput method needs to be slightly different depending on what command caused bytes to be read in the serial reading task.

@RoboDoig agreed that ProcessInput needs to be changed for the new implementation, I avoided doing it in this PR so we could keep it contained to the change from event-based to polling.