Reorganized source tree a bit

All Haskell sources are now beneath a src/ subdirectory, as suggested
by http://www.haskell.org/haskellwiki/Structure_of_a_Haskell_project

Furthermore, I renamed main.hs to Main.hs since that seems to be
conventional, and I already do it for the other source code files.
No need to import Arguments qualified
Made number of parallel HTTP connections configurable

Via a little command line switch