core.time: Conversions and documentation

I got absolutely fed up with having to check core.time's documentation and scrolling through pages of fluff to remember its inconsistent/obtuse conversion syntax, and I decided to fix it.

- ~~You now can convert between `Duration` and `TickDuration` by using constructors (`Duration(tickDuration)` and vice-versa). Isn't this the proper way that all value types in D ought to convert between one another?~~
- The module top documentation now has two tables - one listing the structures and functions in the module, and another showing how to convert between various types.

Here's what the new module documentation looks like:

![screenshot](https://f.cloud.github.com/assets/160894/1927371/21893a18-7e59-11e3-978a-1ecdf8ff0e45.png)

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Needs https://github.com/D-Programming-Language/phobos/pull/1864.