Changes to default webapp configuration from JNDI

I was previously using JNDI path mappings to map
"jndi.config file" to "webapp.config.file" and
"jndi.config directory" to "webapp.config.directory".

There are two problems with this:

1) I was thinking this made sense because the JNDI variable should be
   as straightforward as possible; the JNDI values don't get merged
   with the global config namespace, so they don't need to be prefixed
   to avoid name clashes.

   Upon further consideration, I don't think it's worth having
   essentially two names than mean the same thing. So in the new
   setup, the JNDI variables are also called "webapp.config._" just
   like this config paths. This should be easier to explain and
   remember.

2) In the previous setup, setting the JNDI value "config file" would
   override the config value "webapp.config.file", so if you set both,
   only the one specified by JNDI would get loaded.

   I think this was a counterintuitive choice because it's antithetical
   to the Config library's philosophy of merging configs. As further
   evidence that I was wrong, note that I described it incorrectly in
   the documentation. What I had written in the README and in the
   javadoc on WebappConfigs.webappConfigFactory(ServletContext)
   actually describes the new behavior after this commit.
Merge pull request #2 from chris-martin/jndi-confusion

Changes to default webapp configuration from JNDI