Added support for decimal RPMs.

[ledjohnny]

I've slightly modified the Stepper motor library to add support for fractions of RPMs. I changed the whatSpeed variable to be of type float.

[facchinm]

Hi @ledjohnny , could you move the PR to https://github.com/arduino-libraries/Stepper so we can keep track in the proper repo? Thanks!

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}