Detect Magento if in Sub-Folder (i.e. webroot)

In some configurations the Magento root might not
be the project root. This patch adds a new configuration
detect/subFolders which lists folders to be scanned.

Now n98-magerun.phar can be called from the project root -
just composer, vagrant and other tools.

See also #73
Merge pull request #206 from amenk/feature-detect-subfolders

Feature: Detect Magento in sub-folder (i.e. public, web-root, ..)