Package: Read version from a const

For future releases, just change the version numbers at [this line](https://github.com/am11/WebEssentials2013/blob/0100a86d92d8b4d28d731ee7fba3decb064391e1/EditorExtensions/WebEssentialsPackage.cs#L49) and the [`Source.extension.vsixmanifest`](https://github.com/am11/WebEssentials2013/blob/0100a86d92d8b4d28d731ee7fba3decb064391e1/EditorExtensions/Source.extension.vsixmanifest#L4).