adding ember-cli-meta-tags npm package to provide meta descriptions

[ducks]

refs #425

first pass at adding meta descriptions

This PR adds the ember-cli-meta-tags module. It also adds a head template. This is needed when using multiple modules using ember-cli-head which we now are.

There is an entry for the index route to have a meta description tag at app/routes/index.js.
This can be added to any route by adding an object:

headTags: [{
    type: 'meta',
    attrs: {
        name: 'description',
        content: 'cargo is the package manager and crate host for rust'
    }
}],

this produces:
<meta id="ember823" content="cargo is the package manager and crate host for rust" name="description" class="ember-view">

Feedback and thoughts on other page descriptions most appreciated.

[alexcrichton]

Nice!