Add a SendAtCommand dbus function to just pass a given AT string

This is useful for testing or running custom/proprietary AT commands

For example:

$ qdbus net.sf.nohands.hfpd /net/sf/nohands/hfpd AddDevice "22:22:68:4B:0F:06" "true" 
$ qdbus net.sf.nohands.hfpd /net/sf/nohands/hfpd/22_22_68_4B_0F_06 Connect
$ qdbus net.sf.nohands.hfpd /net/sf/nohands/hfpd/22_22_68_4B_0F_06 SendAtCommand "AT+CHLD=?"
+CHLD: (0,1,2,3)