inveniocfg: new derived config CFG_BASE_URL

• Introduces a new derived config variable called CFG_BASE_URL which is going to contain the relative URL from CFG_SITE_URL.
  
  For example:
  
  CFG_SITE_URL CFG_BASE_URL "http://inspirehep.net" "" "http://someuni.gov/~myuser/www/" "/~myuser/www/"
  
  This is useful when testing single nodes in a multi-host setup with load-balancing, taking advantage of the relative URL lookup of browsers.
  
  All formats and templates should use CFG_BASE_URL whenever an absolute URL is not required.

• Adds the <meta base ..> tag to the HTML header.