Reusability: Move fabric setup to main method.

Because fabric is silenced during module import, when importing bundlewrap
from fabric all output via abort or warn is gone.

By moving the setup of fabric to bw's main method, only when using bw directly
via the CLI this will happen, so for pure users of bw nothing should change.
Move `set_up_fabric` back to `operations` to avoid additional fabric import in cmdline.

Should be moved to those modules which really import operations propably.
Merge pull request #132 from mfriedenhagen/fabric-setup-to-main

Reusability: Move fabric setup to main method.