LZO compression

The user can select LZO compression which is much faster than LZMA. It should definitely get some testing before it is used in production ;-)

- LZO and LZMA bundles can live in the same repository. You can choose each time you do a backup and your choice only affects the new bundles.
- I add a field for the compression type, so the file format is different. Because of the protocol buffers, it should work with existing repositories without any problems and old versions of zbackup should be able to read the files, if they don't use LZO compression. I haven't tested this.
- If liblzo2 isn't available, zbackup will be compiled without LZO support.
- I have implemented some interfaces that can be used to easily add more compression methods. see compression.hh