Fails when Bower is not globally installed.

[rwjblue]

This might seem obvious, but if the bower command is not available in $PATH we throw an error.

Ember CLI bundles bower itself (to avoid just this problem on ember init and ember new), so we should be able to tap into its bower install.

---

Discovered this due to ember-cli integration failures on Ember CLI's Windows CI system. Example output:

     Running: node_modules\ember-cli\bin\ember try:testall
version: 0.2.2--af1623b89b

events.js:85
      throw er; // Unhandled 'error' event
            ^
Error: spawn bower ENOENT
    at exports._errnoException (util.js:746:11)
    at Process.ChildProcess._handle.onexit (child_process.js:1053:32)
    at child_process.js:1144:20
    at process._tickCallback (node.js:355:11)

[rwjblue]

I'm toying with a potential solution for this now...