Fix renamed ISTL namespace backwards compatibility

As @markus.blatt correctly pointed out in #68 (closed), we forgot to correctly build in backwards compatibility when renaming the istl namespace to ISTL. This patch fixes that problem by making sure that you can also include any of the detail headers in the istl/ directory. Moreover, users now get a warning to just include the master header backend/istl.hh instead of all the individual detail headers.

Closes #68 (closed)