Added support for Sketch 45 update system

This closes #31.

.appcast.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<rss xmlns:sparkle="http://www.andymatuschak.org/xml-namespaces/sparkle" xmlns:dc="http://purl.org/dc/elements/1.1/" version="2.0">
+  <channel>
+    <item>
+      <enclosure url="https://github.com/bouchenoiremarc/Sketch-Constraints/releases/download/v1.0/Sketch.Constraints.sketchplugin.zip" sparkle:version="1.0"/>
+    </item>
+  </channel>
+</rss>

Sketch Constraints.sketchplugin/Contents/Sketch/manifest.json

@@ -6,6 +6,7 @@
     "homepage": "https://github.com/bouchenoiremarc/Sketch-Constraints",
     "version" : "1.0",
     "identifier" : "com.bouchenoiremarc.sketch-constraints",
+    "appcast": "https://raw.githubusercontent.com/bouchenoiremarc/Sketch-Constraints/master/.appcast.xml",
     "compatibleVersion": 4,
     "bundleVersion": "1",
     "commands" : [