add login action, link login success with account page, #15

`PostRequestTask` is created for post http request using `HttpClient` and `HttpPost` since the `HttpURLConnection` has some issues (i assumed that it does not follow the redirection) which stuck the right response. 
So all post request shall be send by `PostRequestTask` and the original `RequestTask` is left to do the job with simple `GET`. 
**These need refactoring when the first version is functional.**