Added a new method ErrorReporter#addOnExceptionHandledCommand

Use this method to add commands to be executed before the ErrorReporter
handles the throwable. This can be used to put custom data.
Error reporter: replaced addOnExceptionHandledCommand with setExceptionHandlerInitlizer

as requested in https://github.com/ACRA/acra/pull/87#issuecomment-50932027
Merge pull request #87 from yuriykulikov/for_acra_on_handle_exception_command

Added a new method ErrorReporter#setExceptionHandlerInitializer that sets a Handler that is called when an Exception occurs but prior to logging it.