ID-52

To be in line with the Dashboard 2.0, this PR replaced the data model with Mongoose, and fixed few bugs as well. Like the [ID-32](https://issues.openmrs.org/browse/ID-32).

###In Detail
+  Authorization Code will be deleted after exchanging, or after the expiration time(hard coded as 10 minutes).
+  One (user,client) combination will have only one Access Token.
+  If there is already a Access Token, the consent process will be skipped.(ID-32).