use parameters in main postgresql class

Only default params are set in postgresql::params, the rest is set
in the main class in init.pp. Throughout the module, use the params
from the main class instead of postgresql::params.

These changes are not backwards compatible as global variables
like $postgresql_version are not used anymore.
postgresql: Use dollar in class parameters assignation
postgresql: reorganize class, use stdlib's validate_* commands
Manage dependencies in postgresql's init class
Merge pull request #54 from raphink/parameters

use parameters in main postgresql class