navbar pagenav name should be customizable.

This patch introduces new property 'navbar_pagenav_name', which sets tab name of the current pages TOC.
add property to Customization section
fix typo in the example of conf.py
i18n support for navbar_pagenav_name
Merge pull request #101 from shiumachi/customizable-navbartoc-name

navbar pagenav name should be customizable.