Fix taskdefs for build-single.xml

The classpaths for the taskdefs are outdated.
Add support for OpenCms and Oamp build trees

Separate oamp and opencms build trees. New variables are needed for
this: oamp.input, oamp.output, @{dependency}.buildfile,
@{dependency}.output.zip.dir.
Document build process

Comment alkacon.properties and add build instructions to README
Document build process

Comment alkacon.properties and add build instructions to README
Improve wording in readme