Detect when shell_exec() has been disabled and use HTTP API instead

[mattab]

As of Piwik 2.1.0 we use CLI to issue archiving requests, see: #4610

There is a bug reported in forums: http://forum.piwik.org/read.php?2,112603

when shell_exec is in the list of disabled functions we should not use it, and instead call the API over HTTP as in Piwik <= 2.0.3

[mattab]

In 360ca40: Fixes #4824 Use HTTP API instead of async CLI when shell_exec is disabled

[mattab]

In 68d0bdb: Fixes #4824 fix typo