Enable Windows testing of MemoryFileSystem (#52)

LocalFileSystem exposes a number of bugs in the Dart VM on Windows,
so for now we'll skip those, but this at least ensures that MemoryFileSystem
will have Windows testing, since it has no excuse to not run on Windows.

First step in #51
diff --git a/appveyor.yml b/appveyor.yml
new file mode 100644
index 0000000..09a4a70
--- /dev/null
+++ b/appveyor.yml
@@ -0,0 +1,14 @@
+install:
+  - choco install dart-sdk
+  - ps: refreshenv
+  - refreshenv
+  - set DART_DIR=c:\tools\dart-sdk\bin
+  - set PUB_EXECUTABLE=pub.bat
+  - ps: pwd
+  - pub get
+
+build: off
+
+test_script:
+  # TODO(tvolkert): Broaden scope of tests
+  - pub run test test\memory_test.dart