Add syntax for mercenary CLI engine by @mojombo
Fix serve -> server alias, ensure --drafts is mapped to show_drafts
Switch from commander to mercenary in gemspec
Let jekyll-import specify subcommands and their options for each importer.

@mojombo, this look better?
Check if JekyllImport is defined using Object#const_defined?
If no args are given, then list possible importers.
Merge pull request #1706 from mojombo/drop-commander