JERSEY-1896: Update "bean-validation" section.
JERSEY-1865: Update "media" section.

- PS2: Incorporated review comments. Merged with update of "media" section.

Change-Id: I70148cc965925e9b2aa848628a083cd465c07061
Signed-off-by: Marek Potociar <marek.potociar@oracle.com>
Updated getting started Jersey User Guide section.

-PS2: review changes.
-PS3: rebased, fixed Jersey GitHub links generation.
-PS4: Removed stale TODO

Change-Id: I015bd9779b7472c29a9c29e5d003b42fe1287b8c
Signed-off-by: Marek Potociar <marek.potociar@oracle.com>