Base64-encode user-data in ImportInstance

Fixes TOOLS-488
Fixes GitHub issue #47
Minor help string fixes
euca-import-instance: Base64 encode user data
git revert 894466c27b119ae4672439b9e4ef8e6a1022c1bb
Merge branch 'master' of https://github.com/vasiliykochergin/euca2ools