Fix imports for distribution install.

Running sniffer after installing via pip results in an import error. The cause is leftover relative imports in commit 4c5afa. That commit was created and tested by running sniffer/main.py locally, allowing some imports to succeed that fail when sniffer is run from an installed distribution.

This commit fixes the error by correcting the remaining old-style imports in runner.py and main.py. The imports in main.py are made absolute so sniffer can still be run locally during development.
Merge pull request #12 from wharris/fix_imports

Fix imports for distribution install.