Dummy translation functions and two cases of use

Sometimes, it is not convenient to enclose a translatable string in an actual `gettext`-type function (for instance when it is translated later with specific code), but it must be recognized by automatic translatable string extraction tools. 
In this case a dummy translation function (an identity function) is usually needed. In this case we have implemented `N_` for normal strings and `nN_` for pluralized strings. The utility `pybabel` gets rid of  `N_`  by default and can be instructed to deal with 'nN_'. Also the utility in #238 (basically a `pybabel` interface) takes this into account.
These functions have been used to avoid duplication in the notebook's `javascript` localization mechanism and also to wrap some options of the admin page.