Add workaround for broken modem +CMGR behaviour

Some ZTE modems sometimes issue incorrect +CMGR responses in PDU mode.
This attempts to work around the issues by added a default read status
to received SMS messages. This also includes a unittest containing a
semi-invalid SMS PDU; the encoder/decoder seems to handle it OK (except
for the sending number - might have to update the test and figure out a
way to detect when the PDU has this extra byte issue, and handle it).
Bugfix
Workaround for Wavecom +CPIN response (#19)

Wavecom modems do not issue a terminating "OK" for +CPIN responses. This
lowers the timeout on on the AT+CPIN? check and then inspects the
partially-read response if a timeout occurs; this picks up Wavecom
modems' partial responses.