Add stack support

Required upping the cabal lower-bound, which is, I think,
un-controversial since 1.8 is still pretty low.

.gitignore

@@ -4,3 +4,4 @@ cabal-dev/
 .cabal-sandbox
 cabal.sandbox.config
 tags
+.stack-work

stack.yaml

@@ -0,0 +1,6 @@
+flags:
+  yesod-auth-oauth2:
+    network-uri: true
+packages:
+- '.'
+resolver: lts-3.5

yesod-auth-oauth2.cabal

@@ -8,7 +8,7 @@ synopsis:        OAuth 2.0 authentication plugins
 description:     Library to authenticate with OAuth 2.0 for Yesod web applications.
 category:        Web
 stability:       Experimental
-cabal-version:   >= 1.6
+cabal-version:   >= 1.8
 build-type:      Simple
 homepage:        http://github.com/thoughtbot/yesod-auth-oauth2