Improve cleanup of resources opened by the analyze() methods.

Before issue #8, there would be exactly one FileInputStream per file
that was being analyzed, and it was pretty easy to ensure that this
stream was closed. Issue #8 changed this, but it didn't add logic to
ensure that the extra streams were closed if there was an error before
the document was added to the index.

This change attempts to improve the situation by making sure that
readers or token streams associated with the fields in the Lucene
document are closed if something goes wrong.