ZeroDivisionError for running pyminifier tool on multiple files, one of which being empty

[deeplook]

I found the pyminifier command-line tool to have issues when run on multiples files while one of them is empty, which then raises a ZeroDivisionError, see below. Tested on pyminifier version 2.1 on OS X 10.10, Python 2.7.8 (same on Python 3.4.2):

$ pyminifier --version
2.1
$ touch __init__.py
$ pyminifier __init__.py
# Created by pyminifier (https://github.com/liftoff/pyminifier)

$ pyminifier __init__.py __init__.py
Traceback (most recent call last):
  File "/opt/bin/pyminifier", line 9, in <module>
    load_entry_point('pyminifier==2.1', 'console_scripts', 'pyminifier')()
  File "/opt/lib/python2.7/site-packages/pyminifier/pyminifier.py", line 383, in main
    (float(new_filesize) / float(filesize)) * 100, 2)
ZeroDivisionError: float division by zero

[sejans]

add fix with #51