Refactor to accommodate blank & invalid dates & times

Remove hour & min components in previous commit fecc88db57677869ff23330f3cf5a55d3908c3bb (as they are not currently required)

Adding support to allow string representations of dates/time.

This is to support a non dropdown web form where a user can supply
a date and time in separate input fields, if an error was to occur,
the user input should be represented to the user.

TODO:
* finish tests
* Update version number and documentation.

---

@figwit @bendilley 
```ruby
[4] pry(main)> DateTime.strptime "1/9/2014 13:00", "%d/%m/%Y %R"
=> Mon, 01 Sep 2014 13:00:00 +0000
[5] pry(main)> Time.strptime "1/9/2014 13:00", "%d/%m/%Y %R"
=> 2014-09-01 13:00:00 +0100
```