another round of assorted spelling/grammer/capitalization changes

Some common changes:
--- "middlegame" is a word according to wikipedia, so everywhere I have replaced 'midgame', 'middle-game' and 'middle game' with "middlegame" for consistency sake.
--- Places where code exceeds the "conventional" 80 characters/line "rule", I have split up the lines for better readability.
--- Lots of inconsistencies in trying to represent material criterion, e.g. "king and bishop and pawns vs king" "king+pawns vs king" etc. --> I have tried to make it more consistent. (Mostly in endgame.cpp and endgame.h)

I tried to check and double check my changes, and I hope I didn't make any glaring errors.
My first true pull request. :)