Switch to generic Evernote download url. Fixes #6.

Rather than updating puppet-evernote every time Evernote releases (which is fairly frequently), I'm hoping we can use the URI "https://evernote.com/download/get.php?file=EvernoteMac" to grab the latest. Because the package provider is looking for a URI ending in '.dmg' I've tacked on "&boxen=/evernote_latest.dmg" to the end of the query string which appears to do the trick.

Additionally I've made the source URI a variable which would allow someone to specify a different URI (e.g. to pin to a specific version).

To test with your-boxen, change your evernote mod in Puppetfile to:
```
github "evernote",   "2.0.6.901", :repo => "webbj74/puppet-evernote"
```

And in your manifests either:
```
include evernote
```

or if you want to specify the download URI:
```
class { 'evernote':
  sourceUri => 'https://cdn1.evernote.com/mac-smd/public/Evernote_RELEASE_5.6.0_450741.dmg'
}
```

I also updated the test's Puppetfile and the README.md