Update dependencies

Update code to handle newer versions of github.com/fluffle/goirc
goimports
Fix warnings found by `go vet`

The panic("unreachable") was needed before go1.1.

The log.Printf call was erroneous from the start and should have been
log.Println.
Improve reconnection code

Keep retrying if reconnection fails, with some degree of exponential
back-off.