Introduce includes/utils directory

These are classes that provide facilities for use by any caller, are
independent of user interface, and have a limited set of dependencies on
the rest of MediaWiki. See the README file for a more precise
definition.

These classes cannot go in includes/libs because of a dependency on the
MediaWiki framework, such as wfDebug() or MWException, but they are
otherwise similar. I thought it would be useful to put them in their own
directory, to make them more discoverable, and as part of a general
program of reducing clutter in the base includes/ directory.

I've probably missed a few classes which could be included here, but the
following classes were considered and were rejected for now:

* Fallback: single caller only
* GitInfo: getViewers() has inappropriate dependencies
* HttpFunctions: depends on configuration, $wgTitle
* PoolCounter: depends on configuration
* CacheHelper: depends on IContextSource, wfMemc()

Also moved a couple of classes into libs/ instead, where that seemed to
be more appropriate.

Change-Id: I274cff805b7d694b728a89b764a049cd62d320fe