NEXUS-5807: Add magic bytes to prefixes file

And handle legacy files too. This will require and mean:

a) prefix file will not be loaded if it does not have the "magic"
header (either new one or the old one, which was basically the 1st line
comment generated by Nexus)
b) Central prefixes file should have "## repository-prefixes/2.0 as first line
(or whatever magic we agree on)
c) we should advertise this to anyone who is manually crafting
the prefixes file
Merge remote-tracking branch 'origin/master' into nexus-5807-prefixes-magic
NEXUS-5807: UTs adjusted, MAGIC added
IT improvements by using Java7 try-resources.

no other change (and some reformat)
NEXUS-5807: Fixed IT assertion

Before, @noscrape was at very first line,
now it's after "magic" header.

Also, removed some unnecessary try-catch,
as stream was never assigned.
NEXUS-5807: Applied Alin's comments

Made error message contain the MAGIC to not have
affected users look it up, but have it incorporated
in error message.
Merge pull request #76 from sonatype/nexus-5807-prefixes-magic

NEXUS-5807: Add magic bytes to prefixes file