improve error handling

- show more meaningful messages in connection test dialog
- cleanup method signatures (remove IOException where possible)

Fixes #4
optimize imports
fix coding style
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