Set text property sx-question-mode--data-here

It holds the data of current section, which is a question or answer for
now. This means the data of thing-at-point is always immediately
available through get-text-property.
Define -visit to use answer link if available.
Create `sx-networks` and the associated cache.

sx-network--user-information: User query from site.
sx-network--user-sites: List of sites user is active on for use when
querying sites.
(sx-network--ensure-user): Ensures network-user cache is available for
use.
(sx-network--update): Retrieve most recent network user information.
Generate and update list of favorited questions.

sx-favorites--ensure-favorite-list: List of favorites in format (SITE
QUESTION_ID QUESTION_ID ...).
(sx-favorites--update-site-favorites): Update favorites for given SITE.
(sx-favorites-update): Update favorites for all networks user has an
account on.
Use sx-network--user-filter to include `user_type' property (non-default).
Merge branch 'master' into question-mode-keybindings++
Fix mistakes:

- sx-network--user-information is a var not a function
- remember to setq sx-network--user-sites.
Merge pull request #67 from vermiculus/question-mode-keybindings++

Question mode keybindings++
Refactor sx-network--get-associated to avoid double call to
sx-cache-set.

Fix provides lines (missed quote on symbol).
Fixes as per github comments.

(sx-network--get-associated): Ensure user-sites is also set when
retrieving cache.
(sx-network--ensure-user): Simplify.  Used to ensure cache is loaded
before attempting to use it.

Fix quoted lambda in sx-network--map-site-url-to-site-api.
Fixes as per Github comments

sx-favorites--user-favorite-list: Clarify docstring
(sx-favorites--ensure-favorite-list): Remove extraneous argument
(sx-favorites--update-site-favorites): Change from (list ..) to (cons ..).
GH comment fixes:

Correct file header.
Use sharp quote for function.
GH comment fix:

Use sharp quote for function.
Merge pull request #69 from jleechpe/sx-networks

Functionality for retrieving network information and keeping track of favorite questions.