Genlink -s command should respect proxy settings

This patch fixes issue syncany/syncany#291. HTTPS proxy settings
deposited in the userconfig.xml are applied before making any request
to the syncany API to generate short sharing links.
Circumvent exception when parsing proxy port

Use Ints.tryParse instead of Integer.parseInt since we don't want to do
Exception handling at this point. If there's no port, there are no valid
proxy settings as a consequence. That's it.
Ensure that proxy port is a positive number when creating short urls

Also added some log lines
Merge branch 'develop' of https://github.com/syncany/syncany into hotfix/issue291
Merge pull request #292 from syncany/hotfix/issue291

Genlink -s command should respect proxy settings