Betfairdatabase

Improvements
- Added `clean` method for removing entries with nonexistent files.

Improvements
- New `insert` method for updating the database with new files.
- Object-oriented interface through `BetfairDatabase` class.
- New generic fields: `localDayOfWeek`, `localDayOfWeek`, `localMarketStartTime`.
- New racing-specific fields: `raceId`, `raceTypeFromName`, `raceDistanceMeters`, `raceDistanceFurlongs`.
- `export` method accepts both directories and files as a destination.

Bug fixes
- Decode market catalogue files as UTF-8.

First release.