Additional OAuth2 providers (#1010)

* add google+

* sort signin oauth2 providers based on the name so order is always the same

* update auth tip for google+

* add gitlab provider

* add bitbucket provider (and some go fmt)

* add twitter provider

* add facebook provider

* add dropbox provider

* add openid connect provider incl. new format of tips section in "Add New Source"

* lower the amount of disk storage for each session to prevent issues while building cross platform (and disk overflow)

* imports according to goimport and code style

* make it possible to set custom urls to gitlab and github provider (only these could have a different host)

* split up oauth2 into multiple files

* small typo in comment

* fix indention

* fix indentation

* fix new line before external import

* fix layout of signin part

* update "broken" dependency

vendor/github.com/mrjones/oauth/README.md

@@ -0,0 +1,51 @@
+OAuth 1.0 Library for [Go](http://golang.org)
+========================
+
+[![GoDoc](http://godoc.org/github.com/mrjones/oauth?status.png)](http://godoc.org/github.com/mrjones/oauth)
+
+[![CircleCI](https://circleci.com/gh/mrjones/oauth/tree/master.svg?style=svg)](https://circleci.com/gh/mrjones/oauth/tree/master)
+
+(If you need an OAuth 2.0 library, check out: https://godoc.org/golang.org/x/oauth2)
+
+Developing your own apps, with this library
+-------------------------------------------
+
+* First, install the library
+
+        go get github.com/mrjones/oauth
+
+* Then, check out the comments in oauth.go
+
+* Or, have a look at the examples:
+
+    * Netflix
+
+            go run examples/netflix/netflix.go --consumerkey [key] --consumersecret [secret] --appname [appname]
+
+    * Twitter
+    
+        Command line:
+
+            go run examples/twitter/twitter.go --consumerkey [key] --consumersecret [secret]
+            
+        Or, in the browser (using an HTTP server):
+        
+            go run examples/twitterserver/twitterserver.go --consumerkey [key] --consumersecret [secret] --port 8888        
+
+    * The Google Latitude example is broken, now that Google uses OAuth 2.0
+
+Contributing to this library
+----------------------------
+
+* Please install the pre-commit hook, which will run tests, and go-fmt before committing.
+
+        ln -s $PWD/pre-commit.sh .git/hooks/pre-commit
+
+* Running tests and building is as you'd expect:
+
+        go test *.go
+        go build *.go
+
+
+
+