Support Namespaced Rake Tasks

## Support for Namespaced Rake Tasks

Rake tasks with namespaces look like this:

```rake version:write```

But if you work with Sinatra to create a route with a ":" in it, Sinatra rightly thinks you're talking about a parameter named "write."

At any rate, RakeTaskManager now translates a rake task named ```version:write``` to ```version/write```, and it seems like both parties can live in peace.

## Clarification: tasks + names in TaskManager was confusing

While I was here, I reduced some of the code around TaskManagers and the methods ```names``` and ```tasks```. It used to be that both were used, but ```tasks``` was heavily relied upon.

I eliminated one of them (```tasks```) and now only ```names``` remains.

I think this is simpler: ```names``` returns a list of named tasks (as a list of Strings) and then when someone comes back and asks you it to run a task, they provide a String to ```run```.