Added file read support 

so that a series of messages can be read from a file. For example:

1. Create a file.
2. Stick some messages in it (one per line) and after each message (comma delimited) give the expected return byte size. (e.g. MESSAGE1,3)
3. Use the new -f argument to point to that file
4. simpleserial will iterate through the file and sent all of the messages in the file over the serial connection of your choice.