Subclassing and generalization

Hey there,

Your library got me up and running with this EEPROM pretty quick, but there were a few changes I wanted to make. First, the EEPROM256_512 class should be subclassed into the 256 and 512 devices, and then the mode can be hardcoded for each making code usage easier (since the "mode" of the EEPROM seemed meaningless to me as a user of the library).

Another thing is that the minibuffer size was not completely configurable. In a lot of places a size of 16 bytes was hardcoded, so I made everything depend upon the MINIBUFFER constant.

Test this out and tell me what you think!