DOC: make zipped HTML docs available GH1876
closes #1876

@y-p +1 on this! 
I only have a remark: is it necesary that it is part of the `python make.py html` command? Because now, every time you build the docs locally, it will make a zip (I don't know how long it takes, so don't know if it is annoying). It could maybe also another command that is called when the docs are build for the website?
 It only takes a few seconds on my box and ensures both are in sync but, point taken. 
I moved it into a new `zip_html` subcommand of make.py. 0b9570a
 BTW, it does not work for me (on Windows). The removal fails, and so the rest of the command to create it is not executed (so maybe better split it up).

Also, on Windows it takes around 10s. Always has to be slower :-)
 I anticipated that, hence the blanket try/except. You're welcome to do a PR or just use winzip :)
 Not that important, I won't use it local, so as long as it works for the website it's OK. 
As such, the command works on windows, that's no problem. But because the file is not there yet, removing it fails and so also making the new version because it is contained in one string. 
 That shouldn't be.

```
def html():
    check_build()
    if os.system('sphinx-build -P -b html -d build/doctrees '
                 'source build/html'):
        raise SystemExit("Building HTML failed.")
    try:
        # remove stale file
        os.system('cd build; rm -f html/pandas.zip;')
    except:
        pass
```

That doesn't do nothing if the file isn't there? in what way does it fail, I expected 
the `except:pass` to supress any problems, there might be some stderr noise though.
 zip_html works as well. In general `rm -f no_such_file; do_stuff` will do stuff, it doesn't short-circuit.
is there something else you're suggesting?
 Ah, no, I mean in the `zip_html`:

```
os.system('cd build; rm -f html/pandas.zip; zip html/pandas.zip -r -q html/* ')
```

Here the `rm` fails, so he never gets at the `zip`. The `make.py html` is working fine.
 It seems something else (I assumed the above was the reason), but it's just that the zip command does work via python. `os.system('zip html/pandas.zip -r -q html/* ')`  gives `zip error: Nothing to do! (try: zip -r -q html/pandas.zip . -i html/*)` while running `zip html/pandas.zip -r -q html/*` directly in the command line works perfectly.

But let be, not important, as long as it works for the website.
 That looks like a windows thing. Linux doesn't short circuit on a failed command.
 I assume it is indeed not the failed command, but something with zip. But I am not going to push this to work on Windows.
 so open a PR.... I won't spend time adding a zip dependency, cross-testing on windows
and adding tests, it's just a convenience.... :)
 What I said, not important and I am neither going to spend time on it :-)
 Hurrah for concensus!
