i18n: make gettext gem optional

Require gettext related files lazy and ignore LoadError.
Allow yard to be launched by 'bundle exec' by separating version from yard.rb
Tidy up pull request
i18n: show help message to enable i18n feature
i18n: don't try load .po for the default locale name

The default locale name is nil.
i18n: add locale loading feature to YARD::RegistryStore
i18n: add locale loading feature to YARD::Registry
i18n: support ExtraFileObject contents translation

Attributes are also translated.

Contents are parsed lazy. You can change ExtraFileObject's locale by
ExtraFileObject#locale=.
i18n: add --locale option to "yard doc"

It specifies generated document's locale. For now, extra files are
only translated with messages in po/#{locale}.po.
i18n: add --po-dir option
Add translated document for Japanese
Fix a typo (explcitly -> explicity)
Fix a typo (afformentioned -> aforementioned)
Fix a typo (appered -> appeared)
Fix a typo (unsed -> used)
Fix indentation for markdown
Add --query and --no-modified. Diff now shows modified objects and allows queries
Merge branch 'i18n-make-gettext-optional' of git://github.com/kou/yard
Merge branch 'i18n-support-extra-file-object' of git://github.com/kou/yard
Merge branch 'fix_typo' of git://github.com/yuutayamada/yard