Added more DS3234 support to RTClib

Howdy maniacbug,

Not sure how active you still are here, but I have added a lot of additional support for the DS3234 RTC (used for the DeadOnRTC, https://github.com/sparkfun/DeadOn_RTC). Was thinking it could be good to merge into your repo, and then possibly eventually with Adafruit's (since theirs gets a lot of traffic -- and why not make the extra functionality easier to get?!).

Quick summary of changes here (see also the diffs) are:
* Clock-setting (functions and "set_echo" sketch)
* SPI_MODE1 to SPI_MODE3
* Alarm functions
* CS pin HIGH/LOW
* Housekeeping: *.pde to *.ino in keeping with the Arduino times

If I don't hear from you in a few days, I may just create a pull request straight to Adafruit that would send your improvements their way as well.

And thanks for creating such a wonderful base on which I was able to 

Cheers,

Andy