Add additional functions to Lazy class

[chasecaleb]

Add missing functions to Lazy:

isValid
isValidSync
isType

These are all validation functions that have a value parameter, so it makes sense for Lazy to wrap them.

[jquense]

thanks!

[chasecaleb]

Thanks for merging! Do you think you'll be able to make a release of this on npm soon? That's the last thing preventing me from switching back to using your version.