Improve German translation
Adds tests of simple exporting.
Fixes to export with custom field which is selectable multi items. fixes #46
Uses new rubyzip gem for Redmine 2.4.x or later. refs #43
Fixes translation of 'Export name suffix" for Japanese. fixes #50
Merge pull request #48 from archi-tekt/german_translation_fix

Improve German translation.
Fixes rubyzip version in Gemfile. refs #43
Fixes a deprecation warning. fixes #52
Adds test to show configure page. refs #53
Adds tests default values on Configure page. refs #53
Adds tests to show issues export offset setting (or not). refs #53
Adds tests to change settings on Configure page. refs #53
Fixes regexp of export filename. refs #53
Adds tests to apply changed settings on Configure page to Detailed page. refs #53
Adds tests of quick export. refs #53
Refactors helper methods. refs #53
Fixes a error caused capybara don't wait after click_button. refs #53
Adds simplecov. refs #53
Adds a test to check detailed link in Issues page. refs #53
Adds a test to back Issues page from Detailed page. refs #53