Context: print more information on failed inject

If a required service is not found, we now print more information about
the context and classpath to assist the user in identifying what went
wrong.

Specifically, the list of services known to the context are printed out,
as well as the current thread's classpath's URLs.

Hopefully this will make failed service issues a little easier to
understand.
Merge pull request #21 from scijava/print-inject-failure

Print more detailed information when the Context.inject method fails due to missing a required service. This can be a very serious issue, and simply looking at a stack trace isn't necessarily helpful. Now, we print classpath and context information.