Added isCallable, propertyExists, propertiesExist, keysExist, date, utf8

[terah]

[TomasVotruba]

:+1: for isCallable

[beberlei]

These are too specific for my taste. I am not sure to add them,

[rquadling]

Can you rebase onto master please?

[rquadling]

Also note that some of these may have already been introduced so do not duplicate anything. If the current implementation is in conflict with your implementation, please create separate PRs with reason/argument for the change. This will help me integrate fixes and features separately and more easily.

Thank you.