fixed compiler warnings

several compiler warning fixes as reported in #2
all sizes return unsigned ints now
going from #include <Seb_…> to #include "Seb_..."
exposing radius() in addition to squared_radius()
indentation

(no changes content-wise)
minor fixes

- int to unsigned int
- fixed #include <...> to #include "..."
Merge pull request #4 from hbf/wip-issue-3

Fixes for g++ 4.7 and 4.5 on SUSE Linux 12.2 and 11.3, respectively