Added in code to assign timeslots upon confirmation

* Timeslot is assigned upon confirmation of the account through
  email.
* If the account is from any country besides China, a timeslot will
  be assigned based on a first come, first serve basis.
* Timeslot will be stored in the database using a millisecond format.
  Any part of the application that needs to display a nicely formatted
  time has to parse this millisecond format. This is made easy using
  the joda-time library.
Added a phone number watermarks on the registration pages.

* New WatermarkedTextBox widget that extends TextBox to support
  this function.
* Added in different watermarks for each individual country as per
  the phone number format for each country.
* NOTE: One must apply a cheap hack of calling .setText("") when
  changing the watermarks using event handlers due to some issues
  with the styling. An example of this can be found on RegisterScreen
  line 96.