fixed an edge case with date-changing offsets.
added BIG limitations section to the README.
0.1.4: language changes and a small fix.

- `Copy` is now opt-in. Every `Copy`able type is made to implement
  `Copy`. While unlikely, I haven't deeply thought about
  `Copy`ability so it might change in 0.2.
- `Date::and_date` might behave incorrectly with a large `Offset`;
  this is now fixed.
- Added a BIG (friendly) limitation section to the README.