Initial support for AIX platform.

* Set wlp's shell to /bin/sh as AIX doesn't have bash.
* Set the group to wlpadmin as AIX can't have groups > 8 characters
* Don't install Java on AIX as the java cookbook doesn't support it. We must
  assume that Java is already on the LPAR.
Added 'aix' to supported platforms list in metadata
Added AIX support to server provider
'stop' is a reserved keywords in AIX Korn Shell, so we must use something else.
'stop_service' will suffice; let's rename 'start_service' in a similar way.
Merge pull request #29 from juliandunn/aix-support

Initial support for AIX platform.