add an override to the Error constructor

making message and code mandatory
switch test to use annotations for exceptions

and adding annotation for the error code, drop commented out assertions
update localstorage example for exceptions
reference u2f class directly instead of composer autoload

also drop composer calls from travis
references #11
Add argument type check

More checks will be added soon.
New Exception for user-agent error

This exception is thrown if user-agent passes us error code.
Add autoload.php for non-composer user

This file is originally created by Facebook Team!