📚 Expand package.json ready for gpm

no issue

- Our new marketplace (gpm) will make heavy use of package.json files
- Full details, spec, and validator are coming soon. In the meantime here's the basics:
   - `name` must be lowercase and hypenated
   - `version` must be semver compliant
   - `homepage` should be a valid URL (GitHub URL is ok)
   - `demo` should be a working Ghost install that uses your theme
   - `engines` should contain a Ghost key indicating which version of Ghost the theme is compatible with
   - `license` is required, we recommend `MIT`
   - `screenshots` should provide a relative path or absolute URL to both `desktop` & `mobile` screenshots
   - `author` should be an object. It must contain an email, and can also include a name and url.
   - the `gpm` object should:
	- indicate the package type (`theme` for now, later `app` or `adapter` etc)
   	- list out at most 2 marketplace categories for inclusion. Categories can be found in https://gist.github.com/ErisDS/d0674f57eaa087c9f92cbfc8e984d2d6 but are subject to change.
   - other fields can be included as per a normal npm package, we recommend including the repostory & keywords with `ghost` & `theme` listed.

package.json

@@ -1,4 +1,37 @@
 {
-  "name": "Casper",
-  "version": "1.3.1"
+    "name": "casper",
+    "description": "The default personal blogging theme for Ghost. Beautiful, minimal and responsive.",
+    "homepage": "http://github.com/TryGhost/Casper",
+    "demo": "https://demo.ghost.io",
+    "version": "1.3.1",
+    "engines": {
+        "ghost": "^0.9.0"
+    },
+    "license": "MIT",
+    "screenshots": {
+        "desktop": "assets/screenshot-desktop.jpg",
+        "mobile": "assets/screenshot-mobile.jpg"
+    },
+    "author": {
+        "name": "Ghost Foundation",
+        "email": "hello@ghost.org",
+        "url": "https://ghost.org"
+    },
+    "gpm": {
+        "type": "theme",
+        "categories": [
+            "Minimal",
+            "Personal Blogs"
+        ]
+    },
+    "keywords": [
+        "ghost",
+        "theme"
+    ],
+    "repository": {
+        "type": "git",
+        "url": "git://github.com/TryGhost/Casper.git"
+    },
+    "bugs": "https://github.com/TryGhost/Casper/issues",
+    "contributors": "https://github.com/TryGhost/Casper/graphs/contributors"
 }