add 'other_head' method to Eba_config.Page

This patch enables the developer to add custom elements to the 'head' section,
such as 'meta' tags. It adds an 'other_head' method to 'Eba_config.Page', and
pass it to 'Eliom_tools.F.head' called in 'Eba_page'.
fix forgotten Eba_shared.Page.head_content type in .eliomi
fix missing '~other' parameter for 'connected_page'
Merge pull request #5 from venator/master

add 'other_head' method to Eba_config.Page