Adding Flush() to flush serial port I/O buffers

[madhurjain]

• Current implementation will flush both Input and Output buffers
• Added Flush to Windows calling PurgeComm()
• Added Flush to POSIX calling tcflush()
• Added Flush to Linux using syscall IOCTL with TCFLSH
• Tested on Windows, POSIX and Linux environment

[tarm]

Overall it looks good. It would be great to get some godoc comments about Flush() as well. I think the linux version is what godoc would grab.

http://godoc.org/github.com/tarm/serial