Add the option to use BigEndian for varints.

Makes the library compatible with BitMessage too: see www.bitmessage.org.
network/protocol: use UTC in hardcoded time in test

so that it doesn't rely on a particular location for correct
time parsing.

Signed-off-by: minux <minux.ma@gmail.com>
Merge pull request #2 from minux/patch-0

network/protocol: use UTC in hardcoded time in test
Merge pull request #1 from nictuku/master

Supports BigEndian for varint.