Add JUnit XML support support.

This was provided by the patch found on:
    http://code.google.com/p/shunit2/issues/detail?id=16#c10

I ended up changing the provided patch to not limit it's usage to bash.
Fix the xml included failure messages. Optional environment.

* There was a bug where the echoed message needed quotes to properly work.
* Added the option to include or not the environment into the resulting XML. All you need to do is set `SHUNIT_INCLUDE_ENV` to `0` which is `SHUNIT_TRUE` or `1` which is `SHUNIT_FALSE`.
Missed some quotes.
Added a failing test; if preceeded by $ mkdir reports; export SHUNIT_OUTPUTDIR=reports
then the xml file produced is not xml
Remember to escape & in sed replace; also > is &lt; not %lg; in XML.
Use single method for XML cleanup
Changed message to include an & which also needs escaping
Escape & as well.
Updated documentation to mention xml output; added myself to contributors.
Do not assume date supports %N, but use it if it does.
Allow the test to modify the suite name

the functions oneTimeTearDown and oneTimeSetUp must not be piped. That
is important in order to allow the called code to modify variables such
as the suite name.
Bug fix to avoid appending to existing file

There is no reason to append here. If the file already exist appending
will result in a broken file.
Give possibility to have details about failures

In JUnit xml format, one can have the value of a tag failure populated
with the details, such as the stack trace.