make is.number return false for NaN

[jonhester]

Includes tests

Fix for #72

[arasatasaygin]

Hey @jonhester
Thanks for the PR. What do you think about this syntax?

is.number = function(value) {
    return is.not.nan(value) && toString.call(value) === '[object Number]';
};

I try to write one liners, ease to read.

[jonhester]

Sounds good. Just updated.

[arasatasaygin]

Thanks @jonhester
Merged it.