Fix bug in input.lua, make 0.9-ready, add utf8 editing

1) [input.lua] Pressing backspace while the cursor was at the beggining
   of the text removed the first character. Fixed thanks to riidom.
2) [LÖVE 0.9] Use setLine(Width|Style) instead of setLine. Split
   keyboard.pressed() into keyboard.pressed() and keyboard.textinput().
   (see readme)
3) [utf8.lua] Add support for UTF-8 text editing. May still fail
   spectacurlarly with invalid UTF-8 strings.