NetworkCallBack Interface + General SnackBar Utility (Revised pr)

[laavanyebahl]

Issue no #15 Revised

API exception and type checked while syncing
Interface for NetworkCallBack
Callback implemented in DashboardViewPager
General Reusable SnackBar Utility

[laavanyebahl]

@arazabishov I've triggered the callback as soon as the exception is caught to avoid using the static handleAPIexception method.
setNetworkCallback() could've been added in NetworkUtils and callNetworkCallback() method could've been called by static handleAPIexception() method, but that would require a new instance of the class, leaving the interface uninitiated.