Outsource object signing and validating

The read and write methods for Blob, Tree and Commit no more sign or
validate the byte stream on their own. This can now be wrapped
additionally with the net.secloud.core.objects.signObject and
net.secloud.core.objects.validateObject methods.
Add BlockInputStream.skipToEnd method

Now the net.secloud.core.utils.BlockInputStream does not read a block to
the end automatically anymore and instead the remaining block content
can be skip manually.