new the constructor checks if file has size 0 if it exists. in that case it simply gets deleted...

also added a test for that

…cp-external, so os ssh/scp is used

the library did break when users tried to attach meta data
to the meta data file as it assumed that the target for
meta data is an actual archive entry (which the meta data
file obviously isn't, it belongs to the skeleton of the archive)

this commit also implements checks for double-meta - that
means it wouldn't store the same meta information for any entry twice...

* supporting meta data of meta data
* do not store the same meta data for the same object twice
* improved docs
* additional tests