Fix more-than-one-digit bug

* As the string was not converted into number in
  `Plugin.prototype.updateSelectedCells` function, when the most
  significant digit of larger index was smaller than that of smaller
  index, selected cells were not calculated correctly.

* I replaced `parseInt` function usage with unary `+` operator, which
  also converts string into number.
Add `update` command

* Updating procedure of cells in the table is now separate from
  `Plugin.prototype.init` method into `Plugin.prototype.update`.
* The `update` command can be given by calling the plugin function
  like `$('#selectable').selecTable('update')`.
* When the number of rows and cells of a `<table>` element was
  changed, the `update` command should be given to make the plugin
  work correctly.
* The example code was updated.
Add UglifyJS script

* Minified and uglified version of script can be generated by
  executing `uglify.sh`.
* Minified and uglified version of script was updated.