Make the API more "rustful"

[TheBiggerGuy]

Update the modbus.rs API to make it more "rustful" by:

• Returning empty Results over fixed zero ints
• Better converting result values.
• Not passing in empty mutable values on get_* methods
• Checking explicitly for 0 result values

This is the start of the work I would like to do and not yet completed. I am seeking comments of what you think about the API change?

[zzeroo]

This is definitive a step in the right direction. The changes look good for me.

Could you please select the development branch for your pull requests. Please look at https://github.com/zzeroo/libmodbus-rs/blob/master/CONTRIBUTING.md for the basic rules :)