[fix #51] Correctly return default index file

When the base URL is accessed with no file name, an empty file was
returned. This is due to how the `getResource` method works with
resources in a jar file (actually it would behave the same in any zip
file). If the jar file contains directory entries, an empty stream to
the directory entry is returned, and the `WebLet` think the resource is
found. It is probably safe in \BlueLaTeX to assume that the web client
does not serve any empty static file. Thus, we can filter them out.

Closes #51
Update with breaking changes from sohva

The new version of sohva has some breaking changes in its interface.
Update to the new snapshot version, and fix uses of sohva classes.
Fix paper creation with non default template

The class file was copied before the paper directory was created, which
of course results in some problems that were silently ignored because no
logging was done...
Close inpout stream before opening output stream

On windows a lock is hold on the file as soon as we open it for reading.
Close the input stream before we can open the output stream with no
problem.