Converted all date/time handling and tests to use UTC

Fixes annoying timezone and normalization issues.

For example, this is the output I get when I pick a date on the [API page](http://amsul.ca/pickadate.js/api.htm#method-get-select) and click "*Log the select*" for June 18, 2014:
```
{year: 2014, month: 5, date: 18, day: 3, obj: Date 2014-06-17T22:00:00.000Z, pick: 1403042400000}
```
Take a close look at the difference between "date" and the actual date in the object/pick. I noticed this whilst working on an application and noticed it submitted the dates with a day off. First I thought this was a normalization error but then I realized it was exactly two hours off (my timezone is UTC+2, the Netherlands) and figured it wasn't using UTC dates but local times!

After converting all code and tests to use UTC dates/times everything works as expected and all tests pass. Ran the tests before and after midnight to ensure everything is working as expected. The new API page now gives me the result I expected:
```
{year: 2014, month: 5, date: 18, day: 3, obj: Date 2014-06-18T00:00:00.000Z, pick: 1403049600000}
```
As you can see, the time is set to midnight properly and the date obj/pick matches the others.

It is **much** better and easier to work with UTC values for a date picker, specially when using these values in your application and/or storing it in the database.

Hopefully this can be merged quickly :+1: