Configured Maven to use only local JARs as dependencies.

[odisseus]

The Maven build is now configured so that Maven doesn't download the dependencies from either the Central repository or the Jitpack repository. Maven still needs the network to download its own plugins (e. g. maven-surefire-plugin for testing).

I had to add the dependencies on multibase, multihash and cid explicitly, because the information about transitive dependencies cannot be resolved without POM files. However, this is actually good style, as this project also depends on these libraries directly.

This pull request is related to #215.