Protect API against errant usage.

[mbostock]

The current API does not validate method arguments, coerce types, or make any defensive copies on input or output. This can lead to delayed failures; for example,

• a div might be used for the map container rather than an svg
• a string might be used for the map zoom rather than a number
• the tile size might be set as a number rather than {x: number, y: number}
• the lat or lon attributes of map.center() might be modified directly

Protecting against all types of errant usage might present an undue burden on the library implementors, but we should be able to protect against the more obvious mistakes and make the library more robust. The code size and runtime performance costs will hopefully be outweighed by the benefits of a easier debugging / more predictable failure modes.

Once all input types are appropriately coerced we should also be able to avoid the type-coercing equality operators; i.e., use === and !== instead of == and !=.