Caps break ``target`` option in ``image`` directive.

The following test input::

.. image:: test.png
:target: Target_

A link to the Target_.

.. _Target: http://www.example.com/

Results in the following error output::

test.txt:: (ERROR/3) Unknown target name: "Target".

The first link can not find the name ``Target``, while
it is defined (in fact, the second reference works OK).
Changing the argument of ``:target:`` to ``test`` makes
it work right. Maybe a ``tolower()`` is lacking
somewhere in target handling code.

This situation also happens in the ``figure`` directive.

The test is made under SVN HEAD, but also applies to 0.3.7.