Occur

Occur -- filter lines of current session by a matching term. Does not necessarily require incremental search but using both together makes sense.

See [this screencast](http://youtu.be/RbnkVMJUgJU?hd=1) for how I use it.

If you want to try it out here is my setup code (not included in the PR and requires incremental search):

```js
var occurCmds = require("ace/commands/occur_commands").commands,
    iSearchCommands = require("ace/commands/incremental_search_commands").iSearchCommands,
    occurStartCmd = occurCmds[0];
iSearchCommands.push({
    name: 'occurisearch',
    bindKey: 'Ctrl-O',
    exec: function(iSearch) {
        var options = oop.mixin({}, iSearch.$options);
        iSearch.deactivate();
        occurStartCmd.exec(iSearch.$editor, options);
    },
    readOnly: true,
    isIncrementalSearchCommand: true
});
```