ace.pure : wire up a _CommandManager class

work in progress
missing files from last commit
_CommandManager :  implement `_bindKey` and stub `_parseKeys`
_CommandManager : implement `getCommands` and `removeCommand`

pretty crazy stuff but we are following the javascript impl for now...
_CommandManager : `_parseKeys` throws UnimplementedError for now
Merge pull request #38 from rmsmith/pure_command_manager

pure command manager (work-in-progress)