improve usability for file comments

- display actions in popup-menu
- add actions to edit and remove own comment
- add action to reply (also with short-cut-action "Done")
- add keyboard shortcut "c" to add a new comment
- major refactoring

Resolves #18
fix bug in Gson date deserializer

- hours are in 24 hour format (not 12)
- spotted and fixed by Thomas Forrer
adopt to recent changes in Gerrit extensions-api

Related to issue #56
Merge remote-tracking branch 'remotes/uwolfer/master'

Conflicts:
	src/main/java/com/urswolfer/gerrit/client/rest/http/changes/ChangeApiRestClient.java
	src/test/java/com/urswolfer/gerrit/client/rest/http/GerritRestClientTest.java