Support for default value on DateTimeLocal fields

Currently if you supply DateTimeLocalField with a default value it will not be shown in the web browser correctly. This is due to datetime-local  requires a special format:

1996-12-19T16:39:57 (http://www.w3.org/TR/html-markup/input.datetime-local.html)
Merge pull request #25 from MonkeyDad/master

Support for default value on DateTimeLocal fields