code cleanup, fix bug in saveFile()

Move common functions for getting a filename from the user, reading and
writing text files to common.py to cut down on repetitive code.  Some
basic code cleanup, removing extraneous comments.  Fix bug adding
excessive newlines when writing decompiled text file.
update for adding common.py
update documentation to reflect change in params
Fix a few byte-size problems.

Update length of metadata and metadata size field.  Change plaintext
value for TI-Basic Output( command.  Remove extraneous code, ti_encode
now works within the tiFile object.
add byte value for ']' to ascii symbols