add *.swp to gitignore

Summary:
correctly pass command line params; fix data_dir parsing + test

fix scan() from Key("") + test

Test Plan:
- adapted scanning test for storage.rocksdb
- added test for server.initEngine

Reviewers: spencerkimball

Reviewed By: spencerkimball

Differential Revision: http://phabricator.andybons.com/D53