Documentation tweaks. Search for hg.exe and git.exe on Windoz; hg and git otherwise. Ignore path and just use hg or git as the command if it is found.  Replaced call to subprocess.Popen with subprocess.check_output and calls to os.system with subprocess.call. Added re.DOTALL to -S search in custom view.  Use '~' as the default context for undated tasks.  Make 0 the default outline depth for AGENDA as well as CUSTOM. Removed "break" from several return statements.