Search spellchecker for XAO

As if XAO::Indexer was not cool enough already, it got a spellchecker implementation to suggest alternative search terms (try to misspell something on ISBNdb.com to see the real-life results).

The default implementation is based on GNU Aspell. It includes two basic modes: where Aspell code gets called directly from the searching code, which under mod_perl can lead to large process sizes and under CGI mode can be slow, because it has to load dictionaries every time. The other supported mode is a separate "spelling server" that listens to queries from clients, embedded into the XAO::Indexer search code. This way the server can be even be placed on a different machine from the one actually serving searches.

There is support for building custom dictionaries directly from the data of search indexes to ensure that the spelling suggestions are custom tailored to your search index.

Hopefully I will publish the new code to CPAN and other usual sources in the next couple of days. I need to prepare a release of all other XAO modules that went through some changes as well. Feel free to ping me if this takes longer.