update less task to work as expected

this is still not perfect but more close to what is expected.

In order to have local less files compiled against a default theme, you
would need to provide a `coreDir` variable. This can be provided via
command line or via a file in `grunt/local.conf.json`. This `coreDir`
variable needs to point to a directory containing core less files, like
a distributed version of the core UI or the core UI repository itself.
add empty coreDir to local.conf.default.json

this represents, what the default is
add paragraph for less task

nice to have some place to describe how to work with the `coreDir`
option.
Release v0.2.0