Add CodeMirror.defineOption function for defining new options.

Add findMatchingBracket to the methods exported by this extension.

Closes #746

Prevents styled backgrounds from hiding the selection

It is now more structured (knows about ranges) and allows automatic
unfolding when the cursor enters the folded range.

This allows multiple handlers to listen for a single event, and makes
the API a bit more uniform.

Issue #725

Makes Hebrew text mostly usable

This will make fixedGutter work properly, and make it the only model
(no more gutter that scrolls out of view).

Also reorganizes the basic css file to be easier to modify for users.

Issue #730

Issue #294

Get rid of eolSpanContent mess.

Preparation for RTL text handling.

No more random {x,y,yBot} thrown into the mix.

Experimental for now.

- Multiple gutters are explicitly created with the gutters
  option
- Markers go into a specific gutter, new setGutterMarker,
  clearGutter methods
- The line number gutter is separate
- The 'fixedGutter' feature is removed (painful to do now,
  was shaky anyway)

Closes #738

Closes #753

Closes #751

When dragging there is no src attribute needed on the dummy image.

Though it doesn't seem to have any observable effect on Chrome.

@peterkroon Where does the image actually show up?

It produces a bogus horizontal scrollbar there.

(codemirror2 -> codemirror)

Closes #736

Closes #737

Closes #738