First pass at nginx support

- Automatic detection of whether Docs directories are protected from direct
  file download
- Automatic server detection
- Swap out attachment URLs for links to download script when protection is
  enabled

See #378
Merge branch '1.5.x' into 1.6.x
Use get_attached_file() to build URLs for attachments.

See e159b
Test document for attachment access should be .html, not .txt

Should provide broader support across environments
No-access redirect to wp-login.php
Don't use DIRECTORY_SEPARATOR to filter upload directory

The value of DIRECTORY_SEPARATOR in IIS is '\', which is the MySQL escape
character. As such, it was not being written properly to the database when
recording _wp_attached_file and wp_attachment_metadata postmeta, resulting
in malformed attachment URLs. By using '/' across platforms, we allow IIS to do
its own internal path translations.

See #363

Props @dcavins
Backward compatibility fix for existing IIS URLs. See #363
Coding standards
Overhaul user- and group-specific tag cloud treatment

Previously, these items had been cached in groupmeta/usermeta, and then
reassembled when the page was loaded. This process was complex and full of
bugs, to the extent that it didn't work with users at all.

The new technique involves using WP_Query to pull up associated documents
inline, rather than attempting to cache them. This technique is more reliable,
and shouldn't cause overhead, since the same queries need to be performed later
on the directory page anyway (and are thus non-persistently cached by WP).

See #379

Props @dcavins for an initial patch
Update readme
Show group/user tag filters on group/user Docs directory
Strip query args better for View All Docs link
Better last_activity updating for groups when docs are updated and comments are posted

Fixes #380
Streamline edit styles and scripts
Don't need a separate _edit() last_activity test
Move most edit lock functionality to its own file
Improve edit locks

Use the WP heartbeat API to ensure that users are constantly connected. If
more than 4 pings are missed (default 60 seconds), then the user is assumed
to have closed the browser, and the edit lock is lifted.
Improved redirection etc for simultaneous edits
More consistent styling for Lock toggle
Readme