Added possibility to write an arbitrary screen buffer to display 

i created a new method "displayScreen" which takes an screen buffer "uint8_t* buffer" and writes it to the display. the existing "display" method now uses this new method with the internal screen buffer "screenmemory" as argument.

this enables one to create and manipulate their own screen buffers outside the MicroView library, wich presents whole new possibilities to create art, manipulate on bit level, and so on.

I use it on this sketch: https://github.com/SebiTimeWaster/TV-Screen-for-MicroView

My editor killed millions of trailing spaces (it's best to configure your editor to do the same) which makes the diff harder to read, sorry.
Only because the existing usage cases set a precedent, it's probably good to change it for consistency. (Make sure the MicroView.h is changed as well.)

Thanks.