changed SampleTime datatype

there was a "comparison between signed and unsigned integer expressions"
warning when the library compiled.  I cannot think of any reason why this
have caused a problem, but this fix gets rid of it anyway.
fixed issue #3 and issue #4

-Changed Compute() to a bool from a void,  allowing the calling function
to know when the pid calculation has just been evaluated

-cleaned up the initialization code in the constructor
Changed Example Extensions from .pde to .ino
merge in recent changes on br3ttb/master