Surprisingly disruptive!

Document the ways in which Subprocess is expected to be used.

In standalone execution, ensure event loop exists before child is
created.

Add a missing header.

fixes #7

...and therefore map the set of complete backups to the set of
prunable backups.  This actually turns out to be a simplication
in some ways.

rsbackup now needs (limited) C++11 support.

The prior pruning policy is now available as the 'age' policy.

The inheritable directives are now given their own section.
Within each section directives are now listed in alphabetical order.

This a bugfix; they were always supposed to be inheritable.

Instead of a rather dense if-chain there is now a collection of
classes, one for each directive.  Common checking functionality
is de-duplicated and directive grouping is a bit more coherent.

As with the previous commit this isn't quite a refactoring; at
least one error message is corrected.

Move them to Utils.h and add unit tests.

This isn't quite a refactoring: split() now throws SyntaxError rather
than std::out_of_range when supplied a trailing quoted \, a minor
bugfix.