CGI-Application

Changes for version 4.50

FEATURES
- Better PSGI support in the core
- run_as_psgi() works like run, but directly returns the expected PSGI response structure
- psgi_app() wraps up all the PSGI bits to call and run the application and return a PSGI code ref:
  - $psgi_coderef = WebApp->psgi_app({ ... args to new() ... });
DOCUMENTATION
- docs for header_props() were improved
INTERNAL
- source control management was moved from darcs to git. Those who prefer darcs are are more familiar with it are advised to try the darcs-git.py wrapper for git.
- The test file psgi_app.t is missing from this release and will be added later.

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