Add support to multiple checkboxes inputs.

Example:
<form>
  <input name=foo value=bar type=checkbox checked=checked />
  <input name=foo value=baz type=checkbox checked=checked />
</form>

Now .serializeJSON() should return:
// => Object { foo: Array[ 'bar', 'baz' ] }
Convert string with boolean values to Boolean type and remove wrong logic added to deepSet function in last commit.
Add spec for boolean attributes like checkboxes and radio buttons.
Merge pull request #13 from diaswrd/master

Convert strings with boolean values to Boolean type