BUG: Optional attributes aren't returning `None` when polled

## BUG: Optional attributes aren't returning `None` when polled

Certain attributes in the 4chan API are optional and are not passed in the JSON. When an attribute does not exist for a post, the py4chan library should return the `None` object. 

Going by your code, you've tried to do that with this statement:

    return self._data['sub'] or None

However, it doesn't return `None` as expected when the attribute doesn't exist. It instead gives a `KeyError` that causes scripts to fail in a cryptic manner:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/usr/lib/python2.7/site-packages/py4chan/__init__.py", line 305, in Subject
	return self._data['sub'] or None
    KeyError: 'sub'

For some reason, the FileSize attribute uses this statement that returns `None` correctly:

    return self._data.get('fsize', None)

So I simply replaced the faulty statements with this one in this pull request. Now the error disappeared. 

Why wasn't this one used in the first place?

----

### Alternate Solution

Another solution is to check if the attribute exists with a special `if` statement.

    if 'sub' not in self._data:
	return None
    else:
	return self._data['com']

And now the library works fine.

## Fallback to requests=0.14 using "requests-transition"

When installing the [PyPi package](https://pypi.python.org/pypi/py-4chan/0.1.3) (that I created), pip will also install "requests=1.xx". This version of requests is completely incompatible with "requests=0.14", which py4chan uses, as written in `requirements.txt` .

To make sure the PyPi package installs the old requests version, but also to maintain a path to upgrading to 1.x, I am submitting this pull request with the ["requests-transition"](https://pypi.python.org/pypi/requests-transition)  library as a dependency. (please read the link for more information)

The full solution would be to fix py4chan to use the new version of requests; but of course, we anons are lazy, and the old version works fine.