add several processors - handlers for file operations (tar, zip, 7z)

*reopen PR https://github.com/dizda/CloudBackupBundle/pull/29 - fix tests*
I wanted to add password to archives. But tar command don't have internal possibility of encription, and I decided to add zip compression. The next step was create separate class for compression.

I refactored Database handlers, because file operations not linked with name of these classes (Mysql, MongoDB, Postgresql), in database class only dump operation remains. 

I created Processor classes, which handle all file operations (copyFolders and compress).
In this classes I added password option and compression ratio for archivers.

In documentation I added section about choice of archiver - tar, zip or 7z.

And I fixed database tests and add new processor tests (and catch a bug :) tests rules! )