Parse strings into Time objects for date-related parsing

Users can pass String representation of Times into our [Date-related Liquid filters](https://github.com/mojombo/jekyll/blob/master/lib/jekyll/filters.rb#L27-L71).

Utilizes [Time#parse](http://ruby-doc.org/stdlib-2.0/libdoc/time/rdoc/Time.html#method-c-parse) for flexibility of input. Any values which are valid there are valid here as well.

Fixes #997.