Styleguide: Add guideline for template context variable naming
Styleguide: Add guideline on using double quotes for HTML attributes values
Docs: Add release notes for Pootle 2.5.2-rc1

This will allow to record changes for next release using specific release
notes instead of using the changelog (planned to be removed).
Cleanups on fields
Use JSONP for Google Translate requests

Fixes evernote/pootle#107.
Docs: Small cleanups
Docs: Moved changelog for 2.5.2 to the release notes

First step in the plan to remove the changelog.
Fix zip export

It was broken since 67587450e47fb0d9bd2753edc2a04c5b9f93951c;

* If zip command was not found, subprocess.Popen() will raise an
  exception and the fallback path will not be reached. The old code
  worked because os.system() will just return a non-zero return status.
* If zip command was found, the call will also fail because --out does
  not do what I probably thought it does!