Return Invalid Date / NaN instead of Errors

[cullophid]

This means that our functions no longer break their type signatures when returning errors. or said differently all functions (except parse) are now pure.

[svozza]

We need to remove the references to errors in the type sigs and other parts of the README.

[svozza]

Other than that, looks good. Although, this does convince me even more of the value of the monadic versions of these functions.

[cullophid]

@svozza I tend to agree. Ill look at the docs :)