Allow to set resolution and to compute every time the function is called

First of all. Thanks for a great library. I just started to use it for my balancing robot: https://github.com/TKJElectronics/Balanduino/tree/dev/Firmware/Balanduino, but I wanted to change it a bit for my needs.

This is the changes I have done:
* I added the function SetResolution, this will allow the user to set the resolution to either milliseconds or microseconds.
* By calling SetSampleTime with 0. Compute will calculate the angle every time it is called. This is needed for my robot as I want to update the PID loop as fast as possible.

Also do you mind if I fix the indent of the source code?