Added Login page, basic send, receive data

Added Login page: login & sign up & log out page
Added send/ receive from server side. Click Design under Me to change
ECard info for self. When data need to be saved, click Send. When
retrieving data, click Receive. After changes saved, go back to Me page,
need to click Refresh to see updated info.

Code kind of messy. Needs clean up. The original MainActivity.java has
been changed to Welcome.java, the MainActivity.java is now the login
page, which will be bypassed when already logged in.