Fix opencms taskdef in build-single.xml

The classpaths for the opencms taskdef are outdated and incomplete.
README.md: externaldirectories must be absolute

Instructions edited to reflect that 'externaldirectories' must be an absolute path
Fix com.alkacon.opencms.v8.commons

With commits 7c4c2fff6b610c0f1892252067766f52122d738a and
cda8941c1359e361ac180182b8a14455e504bf57 in opencms-core,
CmsADEConfigData.CONTENT_FOLDER_NAME becomes
CmsADEManager.CONTENT_FOLDER_NAME.
Merge branch 'patch-4'
Merge branch 'patch-6'
Fix com.alkacon.opencms[.v8].comments

Commit cda8941c1359e361ac180182b8a14455e504bf57 in opencms-core renamed
CONFIG_FOLDER_NAME to CONTENT_FOLDER_NAME.

This patch includes 'gallardo/patch-6' as 'com.alkacon.opencms.v8.comments'depends
upon the 'com.alkacon.opencms.v8.commons' module patched there.
Merge branch 'patch-7'