Only allow declares inside declare module (#73)

* Only allow declares inside declare module

* Better error message

test/fixtures/flow/declare-module/8/actual.js

@@ -0,0 +1,3 @@
+declare module A {
+  declar var a:number
+}

test/fixtures/flow/declare-module/8/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Unexpected token. Only declares are allowed inside declare module (2:2)"
+}