Add documentation about the basic 'pymode' setting.

I was looking for a way to disable the plugin altogether when Vim wasn't compiled with Python. I couldn't find any of the settings in the README or documentation.

I noticed in the [source](https://github.com/klen/python-mode/blob/master/plugin/pymode.vim#L5), however, that there was a setting that could disable the whole plugin. I just added this setting to the docs as well as the README.

My ending solution was this in my `.vimrc`:

```vim
if !has('python')
    " Only load Python-mode when Python is enabled
    let g:pymode = 0
endif
```

After using it for a bit, I noticed that loading Python files raised a few Vimscript errors because some of the files were missing a check to see if the `g:pymode` option was 0. My second commit fixes this.