Adding Login for Hawkit.

ParseLogin is called from the HomeActivity whenever there is no CurrentUser set. This behavior will be changed once we add the Profile activity. After profile activity login can be called only when the user wants to check Profile.
One side effect of using ParseLoginUI seems to be that registering a subclass User based of ParseUser results in runtime error. You get a "java.lang.IllegalArgumentException: You must register this ParseObject subclass before instantiating it." in at com.parse.ui.ParseSignupFragment.onClick(ParseSignupFragment.java:174). This is caused when accessing ParseUser user = new ParseUser(). To avoid this will not subclass ParseUser rather just create wrapper around a ParseUser to get similar functionality.
Merge pull request #17 from Hawk-It/loginActivity

Adding Login for Hawkit.