Take the length header into account in Compress() return

Compress was ignoring the length of the byte count varint when
returning the length of compressed data.

This reverts the change in 7642fd8d2dcc2737142e47ecfd8924df7a2b1a0a,
which would only be correct for very short uncompressed data
(<127 bytes).
Fix compressed length calculation with non-zero offsets
Merge pull request #3 from thefactory/fix-compress-length

Take the length header into account in Compress() return