Merge pull request #5 from hi-ban/master

Add an option to switch between the fast scaler and the approximately bilinear scaler.
Center the 320x238 scaler on-screen.
Improve the crispness of detail on scanlines #8 and #10 of 16-scanline blocks in the approximately bilinear scaler.
main.cpp: Do not synchronise the entire filesystem after a forced save, which freezes the emulator until all writes are complete.
manual-en.txt: Document the smooth full-screen option.
Activate the menu with SDLK_HOME.
Modify the manual to reflect the new way to enter the menu.
Merge pull request #1 from Nebuleon/master

update to latest code