Added command description.

Added 'build_clib' and 'build_temp' options (where to put C libraries
  and where to put temporary compiler by-products, ie. object files).
Moved the call to 'check_library_list()' from 'run()' to 'finalize_options()'
  -- that way, if we're going to crash we do so earlier, and we guarantee
  that the library list is valid before we do anything (not just run).
Disallow directory separators in library names -- the compiled library
  always goes right in 'build_clib'.
Added 'get_library_names()', so the "build_ext" command knows what
  libraries to link every extension with.