Change web application to servlet api 2.5 instead of 2.4

[Nicklas Nordborg]

I need to get the "context path" (=the root path of the web applications, eg. /base) in a servlet that is loaded at startup. Servlet API 2.5 added the ServletContext.getContextPath() method. Before this method was only available in HttpServletRequest.getContextPath(). You had to wait for the first request to find out the path.

A side effect of the update is that Tomcat 6 is required. See ​http://tomcat.apache.org/whichversion.html

So. Starting from BASE 2.7, Tomcat 6 or higher is required to run BASE.

[Nicklas Nordborg]

(In [4176]) Fixes #948: Change web application to servlet api 2.5 instead of 2.4

[Nicklas Nordborg]

(In [4177]) Fixes #948: Change web application to servlet api 2.5 instead of 2.4

Added some information in the list of incompatible changes as well.