Add Content-Type handling in get_page() to fix UnicodeDecodeError

This fixes exactly the same issue as in #850 and #810, but much simpler.

When upgrading Sphinx:
```
$ pip install --upgrade Sphinx
...
Exception in thread Thread-4:
Traceback (most recent call last):
  File "/usr/lib/python3.3/threading.py", line 639, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.3/threading.py", line 596, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3.3/site-packages/pip-1.4.dev1-py3.3.egg/pip/index.py", line 286, in _get_queued_page
    page = self._get_page(location, req)
  File "/usr/lib/python3.3/site-packages/pip-1.4.dev1-py3.3.egg/pip/index.py", line 404, in _get_page
    return HTMLPage.get_page(link, req, cache=self.cache)
  File "/usr/lib/python3.3/site-packages/pip-1.4.dev1-py3.3.egg/pip/index.py", line 535, in get_page
    inst = cls(u(contents), real_url, headers)
  File "/usr/lib/python3.3/site-packages/pip-1.4.dev1-py3.3.egg/pip/backwardcompat/__init__.py", line 51, in u
    return s.decode('utf-8')
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x8b in position 1: invalid start byte
```

In this case, the URL it's trying to scrape is http://heanet.dl.sourceforge.net/project/docutils/docutils/0.8.1/docutils-0.8.1.tar.gz, which is obviously not a valid HTML page and there's no actual need for `HTMLPage` to encode and scrape its contents at all.
```
Date: Sun, 24 Mar 2013 16:49:34 GMT
Server: Apache/2.2.16 (Debian)
Last-Modified: Tue, 30 Aug 2011 07:51:04 GMT
ETag: "203350-16e2b8-4abb446046a00"
Accept-Ranges: bytes
Content-Length: 1499832
Connection: close
Content-Type: application/x-gzip
```

Added a few lines to skip invalid URL based on its response headers (whose `Content-Type` is not `text/html`)