Copy full virtual environment to packed charm

[carlcsaposs-canonical]

Enables charm authors to run charm in virtual environment (by modifying dispatch.sh). Addresses #1077

Create symlink to site-packages for backwards compatability

[carlcsaposs-canonical]

Charm authors can use the full virtual environment by modifying dispatch from

#!/bin/sh

JUJU_DISPATCH_PATH="${{JUJU_DISPATCH_PATH:-$0}}" PYTHONPATH=lib:site-packages \\
  exec ./src/charm.py

to something like

#!/bin/sh

JUJU_DISPATCH_PATH="${{JUJU_DISPATCH_PATH:-$0}}" PYTHONPATH=lib \\
  ./venv/bin/python ./src/charm.py