Catch right exception now that we no longer eval
Fix wrong name in SortComparatorProxy
Add support for custom input formats

This adds an InputFormatProxy along the same lines as the other proxies.

Unfortunately, I couldn’t really figure out how to write a meaningful
unit test, so I implemented a (rather contrived) custom input format in
the integration tests to at least make sure the code runs.

A limitation is that the splits returned from the input format must be
standard Hadoop splits, as any input split must be serialisable and
that would require a significant amount of code.
Rename getInstance to wrapInstance