Vendor GYP and libuv into the repo.

Instead of dynamically downloading these as needed during a build, this
checks in those two dependencies directly into the Wren repo. That's a
little lame because users of Wren who aren't building the CLI don't
actually need them, but they aren't too big, so it's not a huge deal.

It makes builds (particularly on Travis) more reliable, because they
don't have to pull down additional content over the network.

.gitignore

@@ -4,11 +4,12 @@
 /wren
 
 # Intermediate files.
-build/
-deps/
-obj/
+/build
+/obj
 /.sass-cache
 *.pyc
+/util/vs2017/build
+/util/vs2017/obj
 
 # I leave a temporary Wren script at the top level so that I can quickly test
 # stuff.