Fix issues #36 and #37 relative to the usage of def.Is

- Get will now panic if called with a bad parameter instead of returning the first object in the container
- The behavior of SetIs changed to avoid calling reflect.TypeOf if the input is already a reflect.Type. It also panics if given a nil interface as its type can not be resolved.

Create new ParentContainer method

Library improvements (#34)

Print panic return values incl. extra information (#26)

Return detailed error (incl. stack trace) instead of `err.Error()` st…

…ring (#25)

Add the Set method to the builder (#19)

Add unshared objects (#17)

The definitions now have an Unshared property.
The unshared objects are not singletons and are rebuilt each time.

Allow service override (#16)

You can now replace a service that already exists in the Builder.

Fix the object closing order (#14)

The objects were closed in a random order. It was possible to have an
object with a closed dependency. It could lead to useless errors (and
error logs). In the worst case the dependency could be a logger and the
logs could be lost.

The objects are now closed in the right order. The dependencies are
closed after the objects using them.

Fix concurrent build calls (#12)

Two requests could build the same object in the app scope.
The first one was used by the first request, while the second one was
used by all the other requests. The first one was forgotten and never closed.
It could happen if the first two Get calls were done at the same time
or/and if the Build function took some time to be executed.