1) Moved basic utility code into base/ directory to make it clear which code
   doesn't depend on Flutter-specific knowldge.
2) Move the CommandRunner subclasses into a runner/ directory because these
   aren't commands themselves.

Fixes #311

wrap up first daemon implementation

Increase the test timeout for the `init` test to 2 minutes (test was timing out downloading assets on appveyor).

remove a runInShell arg on windows when launching pub

I'm trying to get a feel for the code by writing some simple cleanup patches.

Also update tests to be compatible with the presence of iOS and add tests for list and install.

Refactor all the commands to be Commands from the Args package.  Also use CommandRunner for the top-level command.