Rewrite logging in Piwik and remove Zend_Log

Big ideas in this pull request:

- Replaces all Piwik logging classes into one.
- Merges logging channels into one (ie, different files or different database tables).
- Maintains behavior of Piwik where errors/exception are always logged to the screen and errors/exceptions are outputted differently to the screen than to other backends.
- Introduces logging level concept and logging tag concept (see Android logging API).