use plugin name as default artifact id

so we doesn't need both if they are equal

For example:

sonar::plugin { 'sonar-ldap-plugin' :
  version    => '0.1',
  notify     => Service['sonar'],
}
refresh service sonar after install a new plugin

so we doesn't do it ourself

For example:

sonar::plugin { 'sonar-ldap-plugin' :
  version    => '0.1',
}
Merge pull request #6 from sagiru/master

simplify installation of plugins
Do variable access only via @<name> to avoid warnings while puppet apply

For example:
  Warning: Variable access via 'jdbc' is deprecated. Use '@jdbc' instead...
whitespace cleanup