Increase Jenkins requirement to 1.424 so the plugin can build on Java 7.
Use boolean rather than Boolean to prevent NullPointerExceptions.

For example, when the 'useAppVersionURL' field was added, people upgrading the plugin
would start a build, the plugin would try to dereference its auto-unboxed values,
which would be null, causing a NPE.

Also cleaned up unnecessary getter methods.
Prepend ipa/dsym filename with the workspace path, if running on the master server.
Improve documentation and internationalise the plugin.
Report error if a non-integer was given for number of old versions to keep.