Fixed byte/string bug in python3 env

Signed-off-by: Wang <wangwangwar@gmail.com>

When I run "$pygreen gen output" in python2 it's OK, but in python3 env, I got this error:

Traceback (most recent call last):
File "/usr/bin/pygreen", line 5, in 
pygreen.cli()
File "/usr/lib/python3.3/site-packages/pygreen.py", line 157, in cli
args.func()
File "/usr/lib/python3.3/site-packages/pygreen.py", line 152, in gen
pygreen.gen_static(args.output)
File "/usr/lib/python3.3/site-packages/pygreen.py", line 130, in gen_static
file_.write(content)
TypeError: must be str, not bytes

It seems python3 differ byte and string more clearly.
I add a explicit byte->str call and it works both in python2 and 3.