Bug fix for an issue where the parser tries to start a new packet if the...

... checksum equals 0x7E.

Thanks so much for this library! I know you are focusing efforts on the new `xbee-api` project now, but I wanted to contribute this fix for anyone who was still using this code for whatever reason (we still are!).

We found that anytime the checksum byte equaled 0x7E, our `data` event listeners weren't firing, due to the fact that 0x7E is the start delimiter. I simply added a flag to make sure a new packet starts only if the 0x7E value isn't escaped.

Thanks again!