Handle requests using timestamps

[Andersama]

Changes the return types of a few functions to pass along a timestamp to blockTillConversionComplete.
A few overloads are added to blockTillConversionComplete.

Example usage:

sensors.setWaitForConversion(false);
DallasTemperature::request_t request = sensors.requestTemperatures();
// do other work
sensors.blockTillConversionComplete(sensors.getResolution(), request);
// or
sensors.blockTillConversionComplete(sensors.getResolution(), request.timestamp);

Note: The struct isn't strictly necessary. I figured I should make each request function return the same struct to avoid confusion. requestTemperatures() could easily return just an unsigned long. I added a conversion operator to bool so that on the off chance anyone is using an older library their usage shouldn't break, I left the bool first for any binary compatibility issues for similar reasons. I'm not particularly happy with how long DallasTemperature::request_t is compared to unsigned long. I was considering result_t over request_t, but that's simple enough to change and obviously feel free to edit.

I added one new overload for the new request type which isn't strictly needed, but I figured would be nice to have / a good example of how the request struct can be used. It specifically handles the case where the request struct indicates the probe was disconnected when the request was made to avoid any unnecessary wait.