(Maint) Small docs fixups

The docs for how to use the BindingsFactory where slightly wrong and
indicated you had to create an instance. The factory methods are all
class methods on the BindingsFactory and so instance is needed.
Add a lookup() function for injected values

The lookup() function will use the configured bindings and injector to
lookup a value. If one is not found, then nil is returned (which is
undef in the puppet language). Any errors from the injector are
propogated out to the caller.
Merge pull request #30 from zaphod42/feature/binder

Add a lookup() function for injected values