fix possible error after plugin installation

- plugin got disabled (jdom exception because of setting null values)

Resolves #31
fix build with jdk6
prevent possible NPE

- author may be null (for deleted users)
- author name may be null for system messages
proxy inner class must be static
refactor review notifications

- show notifications at startup when enabled (even when Gerrit tool-window has not been opened)
- allow notification setting updates without IDE restart
- do not reload main grid automatically (open reviews and selection may be lost otherwise)
- move things into own class
disable warnings about unregistered components

- components are set up with proxy classes
implement filter for changes panel

- move toolbar to top of changes panel
- add filters for:
    fulltext, status, branch, owner, reviewer,
    starred changes

Resolves #21
Merge pull request #1 from forrert/features/filterpanel

Features/filterpanel