Updated is Log class to store files in subfolders (year / month)

I think current logging system is not that optimized, especially if you keep logs for an year+ (see attached image x 12 months or more):

![logs-folder](https://f.cloud.github.com/assets/223750/64176/53ecb1de-5e33-11e2-8c96-20e0edb6258c.png)

Suggested change in code stores files the following way:

- /2012/01/...
- ...
- /2013/01/...

We could make it optional with one more config item in config.php