Allow service calls with empty requests

std_srvs::Empty has a request message of size zero. SerialClient.send
returns the size of the sent message, which is checked to ensure
data crossed the serial line. Accommodate services with empty requests
by modifying the check to acknowledge all transmissions of zero or
more bytes as valid.
Fix SeviceServer member names in error message

'm' prefix was omitted, causing an exception while trying to print
an error about md5 mismatches. Fix this to allow the error to be
presented to the user.
Merge pull request #51 from spencerjackson/emptyServices

Allow services to have empty requests