not enough freespace error message

Small feature which checks the available disk space before starting a new task. This way, if the disk Cuckoo is running on runs out of memory, we don't start analysis and don't get nasty exceptions all over.
The minimum size is configurable (in MB).

Two improvements might be necessary:

* Windows support (for linux and macosx, `os.statvfs()` suffices.)
* Multiple VM support (i.e., check the minimum free disk space multiplied by the amount of configured/running VMs - needs some extra thoughts with regards to the VM tags stuff.)