Support for PEAR package

Hi Facebook team,

PEAR's package.xml is complex, to support PEAR package auto-generation, 
I added a package.ini config file for building PEAR package.

By using Onion PEAR package builder, 
this package can be distributed through PEAR channel server, and use PEAR or Pyrus installer to install.

onion.phar is a tool for building package.xml from package.ini:

      onion.phar -d build

By running this, package.xml will be generated and a package file Facebook-{version}.tgz will be there. 

To distribute the PEAR package file with PEAR channel server, there is `Pirum` doing this.  

Thanks. :-)