Refactor common network byte order decode functions to utility class

Refactor common network byte order decode functions to utility class

We frequently need to read a network byte order value from a byte[]
buffer, but we don't want to pay the penalty of creating a ByteBuffer
and using the stock J2SE big endian integer decode routines each
time we need to access such data.

The new "NB" utility class holds the most tuned versions of these
conversion methods that I have been able to create, and makes them
available for reuse throughout all of jgit.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>