Xlogparse

I'm interested in parsing WAL files and processing the results programatically.  Your work on the xlogdump tool provides a great platform, and this pull request factors out some of the prime logic into a stand-alone library so arbitrary clients can customize their handling on a record-by-record basis.

Please let me know if there's any changes you'd like to see before merging, or if any other information would be useful.

thanks,
   - peder