bitbake: add crate fetcher

This adds a handler for crate:// URLs and rewrites them to crates.io
https endpoint and handles properly unpacking the crate. This fixes #24.

classes/crate-fetch.bbclass

@@ -0,0 +1,13 @@
+#
+# crate-fetch class
+#
+# Registers 'crate' method for Bitbake fetch2.
+#
+# Adds support for following format in recipe SRC_URI:
+# crate://<packagename>/<version>
+#
+
+python () {
+        import crate
+        bb.fetch2.methods.append( crate.Crate() )
+}