Console API rework

[zsparal]

This still has the names OffscreenConsole and RootConsole. I'm kind of partial to these, since you can import them nicely from the console module and still know what they do.

If the naming is fine as it is, then this closes #137

[tomassedovic]

I'd prefer Offscreen and Root names in the console module. If someone wants the imported names to be more explicit, they can always say use tcod::console::Root as RootConsole.

And we can do that re-export in lib.rs, too. What do you think?

Otherwise, this looks great. For the record, making Console::con unsafe makes sense and I actually like that it's public -- people can use it to call the low-level FFI stuff if the want (which is inherently unsafe, too).

[zsparal]

Yes, I like the public Console::con as well, but it would've been more elegant if I could've implemented it using the AsNative traits. But it's not really feasible if we want to keep the current blit semantics (which are more important than the unsafe Console::con).