[Review]: Change JSOCClient API to match that of the Unified Downloader / VSO Client

This change makes the `JSOCClient` act like the VSO / Unified Downloader, in that you make a query that returns a list of results that you can then download.
To demonstrate this I copy some documentation below:


```python
    Query JSOC for some HMI data at 45 second cadence:

    >>> from sunpy.net import jsoc
    >>> client = jsoc.JSOCClient()
    >>> response = client.query(jsoc.Time('2010-01-01T00:00', '2010-01-01T01:00'),
    ...                         jsoc.Series('hmi.m_45s'))

    the response object holds the records that your query will return:

    >>> print response
            DATE         TELESCOP  INSTRUME  ... WAVELNTH     WAVEUNIT
    -------------------- -------- ---------- ... -------- ---------------
    2012-09-05T08:27:19Z  SDO/HMI HMI_FRONT2 ...   6173.0 Invalid KeyLink
    2012-09-05T08:27:20Z  SDO/HMI HMI_FRONT2 ...   6173.0 Invalid KeyLink

    You can then make the request and download the data:

    >>> res = client.get(response)
    Request JSOC_20140724_947 was submitted 6 seconds ago, it is not ready to download.
    Request JSOC_20140724_947 was exported at 2014.07.24_22:08:09_UT and is ready to download.
    2 URLs found for Download. Totalling 30MB

    This returns a Results instance which can be used to watch the progress
    of the download.

    >>> res.wait(progress=True)

    *Example 2*

    Query the JSOC for some AIA 171 data, and seperate out the staging and the
    download steps:

    >>> from sunpy.net import jsoc
    >>> client = jsoc.JSOCClient()
    >>> response = client.query(jsoc.Time('2014/1/1T00:00:00', '2014/1/1T00:00:36'),
                                jsoc.Series('aia.lev1_euv_12s'), jsoc.Segment('image'),
                                jsoc.Wave(171))

    the response object holds the records that your query will return:

    >>> print response
            DATE         TELESCOP INSTRUME          T_OBS          WAVELNTH WAVEUNIT
    -------------------- -------- -------- ----------------------- -------- --------
    2014-01-06T15:07:12Z  SDO/AIA    AIA_3 2013-12-31T23:59:36.34Z      171 angstrom
    2014-01-06T15:07:12Z  SDO/AIA    AIA_3 2013-12-31T23:59:48.34Z      171 angstrom
    2014-01-07T15:05:10Z  SDO/AIA    AIA_3 2014-01-01T00:00:00.34Z      171 angstrom
    2014-01-07T15:05:10Z  SDO/AIA    AIA_3 2014-01-01T00:00:12.34Z      171 angstrom

    You can then make the request:

    >>> requestIDs = client.request_data(response)
    [u'JSOC_20140724_952']

    This returns a list of all the request identifiers for your query.

    You can then check the status of the request, which will print out a status
    message and return you the status code, a code of 1 means it is not ready
    to download and a code of 0 means the request is staged and ready. A code
    of 6 means an error, which is commonly that the request has not had time to
    get into the que.

    >>> status = client.check_request(requestIDs)
    Request JSOC_20140724_955 was submitted 10 seconds ago, it is not ready to download.

    Once the status code is 0 you can download the data using the `get_request`
    method:

    >>> res = client.get_request(requestIDs)

    This returns a Results instance which can be used to watch the progress
    of the download.

    >>> res.wait(progress=True)
```
I just meant that ``if segment != '':`` is the same as ``if not segement``