strengthen serial communication & reset

[bganne]

Hi, thanks for the great work with weewx-meteostick, it works really well.

I only had an issue during initialization: my meteostick was already sending data over serial in human readable format when I started weewx, and weewx-meteostick failed in 2 different ways:

1. failed to decode some bytes: it looks like during reset, some junk bytes can be read from the serial port. I fixed this issue by adding the 'ignore' parameter to the decode() function used during reset
2. the reset command does not always work: I fixed the issue by adding an additional newline character before issuing the reset command so that if there were some character already written it is discarded. I am also making sure the output buffer is reset beforehand and I forced it to be flushed afterwards

On top of that, I also switch encode/decode from utf-8 to ascii. Not strictly necessary but I think it is more correct in this case.