First cut of Capistrano 3 version

Working with a local project, no specs or doc updates yet
Tweak gemspec, elide specs which don't work with Cap 3 :(
Update readme for Cap 3 and new conventions
Use error-level logging for error
Rename to capistrano/deploytags
Merge pull request #22 from mydrive/cap3

Rewritten for Capistrano 3