Refactoring

Changes:

- Trailing invocation support, fixes #2.
- Removed `max` argument, ranted about it [here](https://github.com/component/throttle/issues/2#issuecomment-52077563) :).
- Updated and added new tests.
- Updated readme with example and API description.

Throttle now caches last context and arguments to apply to function on invocation, and than clears them to not introduce any memory leaks.

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Sorry that trailing and max removal aren't separate PRs, but couldn't image how to accomplish that without merge conflicts and overall chaos :)