em32dx-a e-paper commands

[ochristensen]

add 33 additional commands supported by samsung's e-paper app

a couple oddities:

• brightness has a separate command for epaper - i'm guessing because it doesn't really mean the same thing?
• poe support is in the "supported features" response but seems to be hardcoded to false...despite there being a poe+ command?
• a couple "set only" commands are technically getters, ie GET_THUMBNAIL (to show what's currently on the display): you set a url to a server you create and they POST a response back with the data

my title might be slightly misleading as i don't know how many of these commands are only applicable to e-paper displays (and mine in particular.) the brightness certainly is given there's already a command for that that doesn't apply to mine, and i've named gamma and background color as such as well.