Add support for AT25 series, reduce memory footprint of AT24 series. 

* Added support for SPI eeproms with AT25.js module. 
* Removed support for the i2ca_ argument with all AT24 calls. This was meant to allow use of multiple eeproms with a single instance of the AT24 module. The problem with this was that a) it didn't save much memory even in that use case. b) it used more memory in the most common use case and c) if the EEPROMs have different pagesizes, you can get into bad places if you use it. 
* Renamed AT24.arrayToString() to AT24.aToS() to reduce memory footprint by a few JSVARs.