handle: enumerate typ with int instead of string
att: move att related code to a separate file
att: add a helper function for error response
extend the end group handle of the last service to 0xFFFF

This could save one query during the primary service discovery
procedure.

Quoted from spec:
This sub-procedure is complete when the Error Response is received and
the Error Code is set to «Attribute Not Found» or when the End Group
Handle in the Read by Type Group Response is 0xFFFF.
l2cap_writer: rename Write*() to Write*Fit()

go vet expects the Write*() have same signature as bytes.Write*().
So we rename our variant consistently to Write*Fit().
fix typo of the format argument
Merge pull request #5 from roylee17/feature/adhoc-changes

Feature/adhoc changes
git: ignore sample executable
l2cap: remove unnecessary type conversion
notifier: extract notifier code to a separate file
server: remove singleton constraint from Server for now

We could have multiple instances (one instance per HCI
device).  Associate the serving status on per-server
basis instead.
conn: move conn code to a separate file
reorg: remove l2capHandler
prepare to shift gatt handling from l2cap to conn
shift gatt handling from l2cap to conn
decouple l2cap and conn(gatt handling) with io.ReadWriteCloser
prepare to shift gatt handling from server to conn
shift gatt handling from server to conn
move notifier from characteristic to conn
move handles from l2cap to server