Incremental dumps/TODO

Required for production deployment

 * fix bug with creating diff dumps for stub dumps; make sure diff dumps for current dumps work correctly
 * code review
 * integrate with xmldumps-backup
 * reasonable handling for file-related errors (file not found, access denied, etc.); right now, the application just crashes
 * Tracking bug

Later

 * better compression for metadata and indexes
 * compress indexes using varints (or LZMA?)
 * add index for named users
 * compress list of revision ids of a page using varints
 * store parent id as a varint difference from revision id
 * add index for namespaces
 * see if it's possible to tweak LZMA (language-specific Huffman tables?)
 * library for binding with other languages