Extend the Arcrepository interface to include the 'endoffset' argument in the get method

The NetarchiveSuite method ArcRepositoryClient.get(String arcfile, long index) only includes the startoffset in the argument list.

This is OK, because we assume that the archive knows what a WARC or a ARC files is, and can read a (W)ARC-record correctly.

This assumption is not true, if we want to access data uploaded to a bitmag-style archive (https://sbforge.org/display/BITMAG/The+Bit+Repository+project).