Validate arguments to reduce the chance of obscure error messages

[blancas]

This is a follow-up on #17.
The heavy use of nested closures and high-order functions results in arguments to a function being used far away from their source. When there's an error, looking at the stacktrace doesn't tell much of what's happening.

Validating arguments when possible provides a fail-fast (or faster) that should be better than the current situation. Sounds like spec could be useful here, but this lib shouldn't require 1.9.