Check that all required nodes exist for plugin description

[NateWr]

Aside from the checks on release packages, the following nodes should be required in each plugin:

• One <name>.
• One <homepage> with a URL that resolves with a 200 http status.
• One <summary>.
• One <description>.
• One <maintainer> with:
  • One <name>
  • One <institution>
  • One <email>

@asmecher Where we specify locale, do we require it to be en_US or at least one of the nodes be en_US?

[kabaros]

These should already be enforced as we're running the schema validation at the first step of the build

This is the link to the schema http://pkp.sfu.ca/ojs/xml/plugins.xsd

[NateWr]

D'oh!