fix http client warnings (when using self-signed certs)

- message: "SimpleHttpConnectionManager being used incorrectly.  Be sure that HttpMethod.releaseConnection() is always called and that only one thread and/or method is using this connection manager at a time."
fix possible NPE
add some troubleshooting hints for common problems

Related to #29
Related to #39
show message when Gerrit plugin 'download-commands' is not available

Related to #39
do not show fetch action in UI

- no real use-case for it
- used internally for compare branch and cherry-pick actions

Resolves #40
cleanups

- remove "my" prefix from some fields
- optimize imports
Merge remote-tracking branch 'origin/master' into intellij13