Inject package version, files into nuspec on build

* Move /build/ -> /pkg/
* Add nuspec
* Refactor package:nuget workflow to create a temporary directory to build the package from - so we can store the transformed file
* Add grunt-template task to transform the nuspec

pkg/nuget/package.nuspec

@@ -0,0 +1,20 @@
+<?xml version="1.0"?>
+<package xmlns="http://schemas.microsoft.com/packaging/2011/08/nuspec.xsd">
+        <metadata>
+                <id>Archetype</id>
+                <version><%= version %></version>
+                <title>Archetype</title>
+                <authors>imulus, tomfulton</authors>
+                <owners>imulus, tomfulton</owners>
+                <projectUrl>http://github.com/imulus/archetype</projectUrl>
+                <description><![CDATA[Archetype]]></description>
+                <tags>umbraco</tags>
+                <iconUrl>http://github.com/imulus/archetype/raw/master/assets/logo.png</iconUrl>
+                <licenseUrl>http://www.opensource.org/licenses/mit-license.php</licenseUrl>
+        </metadata>
+        <files>
+                <% for (var file in files) { %>
+                        <file src="<%- files[file].path %>" target="<%= files[file].target %>" /> 
+                <% } %>
+        </files>
+</package>