Expose injector, bus and commands publicly

Avoids to be forced to inject main app class to get these elements
initialized by the Dawn class.
Added support for existing instance in inject() method

When calling inject() with an instance that has been manually created,
inject its dependencies afterwards, without the need to permanently
keep the mapping configuration to this particular instance.
Merge pull request #30 from avangel/c762e9ab84e926fd4539867abb13702dadd4bb63

Manually created instance injection