Use processing-java to run sketches on OS X

From Processing 2.0 beta 5, the 'use external editor' option has been removed.
Instead, a processing-java command is supplied to run sketches from the command
line. This commit updates vim-processing to use processing-java instead of the
previous AppleScript tool. processing-java can be installed by choosing 'Install
processing-java' from the Processing tools menu.
Restore bin/runPSketch.scpt

Restoring bin/runPSketch.scpt deleted in b2d864677a3f451a2bf6f38d837847754365406a, as this file should be retained for backwards compatibility.
Run sketches on OS X using AppleScript by default.

To preserve backwards compatibility of vim-processing on OS X, use of
processing-java is now user configurable, with the original AppleScript
utility used by default. To enable processing-java on OS X, set
g:use_processing_java. processing-java is used by default on other
platforms.
Update documentation.

Update documentation to explain new behaviour of processing-java for
running sketches.
Add note on adding processing-java to path.
Fix typo in g:use_processing_java variable name.
Merge pull request #4 from vortura/master

Use processing-java to run sketches from Vim.