Add tab completion to REPL
Refactor and tab completion

Tab completion lives in it's own module. The completion is extensive
enough to cover namespaces and vars, as well as Ruby constants, classes
modules, and singleton methods.
Current namespace in tab completion

Also clean up comments and refactor specs.
Include symbols from referred namespaces
Merge pull request #31 from noprompt/readline

Add tab completion to REPL