Add the behavior for `blur` event

* I added an event handler for `blur` event of the table which
  deselects the selected cells.
Refactor code

* The plugin object is now accessed in event handlers by calling a
  function named `findSelecTableObjFromElem`.
* The procedure that disables user select is wrapped into a function
  named `disableUserSelect`, which accepts a DOM element or jQuery
  object as the parameter.
Fix wrong behaviors of `blur` and mouse events

* I modified the handler for `blur` event to

    * set `isMouseDown` flag to false so that moving the mouse cursor
      inside the table does not select the cells
    * set `$cellA` to `$()`
    * trigger an event named `selectionupdate.selecTable` with extra
      parameters than contains empty array of selected cells and
      undefined last selected cell

* I modified the handler for mouse events to disable user select of
  document when selecting the cells and enable it again when selecting
  ends.
* The example code was updated to deal with the change in the behavior
  of `blur` event.
Modify the way of string-to-number conversion

* I modified the way of string-to-number conversion from using unary
  `+` operator to using `Number()` function.
* This modification was made because using `Number()` function is
  much faster than using unary `+` operator in Mozilla Firefox. See
  [jsPerf benchmark](http://jsperf.com/parseint-vs-unary-operator/7).
Modify uglifying script and update uglified code

* I modified the filename to which uglifying script outputs the
  uglified code into `selectable.min.js`. (all lowercase)