add feature to show line based file comments

- Gerrit 2.7 is required because of missing REST method in 2.6
Merge branch 'master'
initial version of file based comments

- adding works
- removing of newly added, but not yet submitted, comments works, but ui does not update (click in icon gutter)
improve error handling

- do not fail loading comment when Gerrit is too old (i.e. <2.7) (just returning empty)
- fix UnsupportedOperationException on Git error
fixes for file comments

- prevent possible NPE
- remove gutter icon after removing comment
Merge remote-tracking branch 'remotes/origin/master' into features/file-comments

Conflicts:
	src/main/java/com/urswolfer/intellij/plugin/gerrit/ui/GerritToolWindowFactory.java
Merge remote-tracking branch 'remotes/origin/master' into features/file-comments

Conflicts:
	src/main/java/com/urswolfer/intellij/plugin/gerrit/rest/GerritApiUtil.java
	src/main/java/com/urswolfer/intellij/plugin/gerrit/rest/GerritUtil.java
add support for file based comments (view and add)

- Gerrit 2.7 or newer is required for viewing comments

Resolves #3

Merge remote-tracking branch 'features/file-comments'