emacs mode enhancements

Fixes for emacs mode.

1. setMark
    An approximate emulation of the highlighting emacs does in transient-mark-mode. C-space sets the mark, and subsequent keyboard cursor movement commands select the region between mark and the new cursor position ("point") instead of just moving point.

    Inserting text or clicking the mouse in the document clears the mark and the current highlighting. C-space C-space (two setMark commands in a row at the same point) clears the mark. 

2. killLine
    Works more like it does in emacs. Kills from point to the end of the line. If the line is already empty, it kills the line entirely. So to completely remove a line and close up the space, do C-a C-k C-k. (TBD: C-a needs to be fixed to work more like emacs --- it should always move to the actual beginning of the line even if the line begins with white space.)

3. forward-word and backward-word
    Fixed to move by "long" words, i.e. spaces between words should not be considered "words". (Shouldn't this be the default for Ace as a whole? I don't understand the point of "short" word movement.)