Restore the default signal handlers for child processes.

Python sets up default handlers for several signals--which includes
ignoring SIGPIPE.  Unfortunately, these are inherited by the child
process.  When using 'diff-highlight | less' as the pager, SIGPIPE will
be ignored by diff-highlight and will try to keep reading from stdin.
If you try to exit less with the 'q' command, but have a big Subversion
operation running, then we continue to pipe data from Subversion to
diff-highlight forcing you to ctrl-c to terminate the operation.

Instead, let's restore the default signal handlers in the child
processes.  Not only does it provide a cleaner environment for the
children, it helps fix the above situation.
Version 0.6.3:

- Restore default signal handlers in pager.

- Add config option "useShell", defaulting to False, that allows use
  of the shell for invoking the pager.  Using the shell can be a problem
  if the supplied "cmd" is invalid.  Since the default "cmd" is "less",
  this doesn't play well on Windows where "less" is unlikely to be
  installed.  The shell behavior is still present, but you have to ask
  for it explicitly now.