Please update installation guide

If I clone this projec then run the zeronet.py, it doesn't work.

Even after I run `sudo pip install -r requirements.txt`, It doesn't works.

```
beenotung:ZeroNet$ ./zeronet.py
- Starting ZeroNet...
ERROR:root:Unhandled exception: No module named gevent
Traceback (most recent call last):
File "./zeronet.py", line 18, in main
import main
File "/home/beenotung/local/opt/ZeroNet/src/main.py", line 9, in
import gevent
ImportError: No module named gevent
Traceback (most recent call last):
File "./zeronet.py", line 91, in
main()
File "./zeronet.py", line 64, in main
traceback.print_exc(file=open(config.log_dir + "/error.log", "a"))
AttributeError: 'Config' object has no attribute 'log_dir'

```

Maybe related to https://github.com/HelloZeroNet/ZeroNet/issues/1056