Add .war extension to _map_jar

- this allows 3rdparty .war dependencies to be bundled into
  an app bundle.  You can use this to serve 3rdparty
  web applications from a jetty container server, and
  have this managed via ivy.
Merge pull request #23 from slackhappy/fs-nonbeta-warsupport

Add .war extension to _map_jar
Add PANTS_CONFIG_OVERRIDE to allow a second config file

* PANTS_CONFIG_OVERRIDE specifies a buildroot-relative config file path
* An env variable was chosen because pants.ini is loaded before argument
  parsing
* That file is loaded along with pants.ini
* If a value is in the override file it is chosen instead of pants.ini
* There's no cross-referencing between the files, so each file must be
  internally consistent
Merge pull request #24 from jeffjenkins/master

Add PANTS_CONFIG_OVERRIDE to allow a second config file
More documentation for the overrides feature
Merge pull request #25 from jeffjenkins/master

More documentation for the overrides feature
Merge remote-tracking branch 'twitter/master'