Add ability to run compiled application

- Ctrl+F7 will run compiled js application in user-specified
  browser.
Add build task to run preferred browser

Adds a new settings, 'dart_user_browsers', that enables users
to specify multiple non-Dartium browsers, per OS.

If any browser is specified as 'default', it will be used when
running web apps via Ctrl+F7.
Check for path erros
Require 'dart_dartium_path' to be set

- Fixes for OS X
Remove comment
Comments and docstrings
Add per-OS setting to locate Dartium

Improve xplatorm compatibility.
Improve OSX compatibility
Merge pull request #5 from my-personal-forks/fixes-osx

Improve OSX compatibility
Improve OSX compat
Assorted fixes
Add comment
Abort is no default user browser is defined
Merge pull request #232 from my-personal-forks/smart-build-fixes

Add ability to run compiled application