* Added topic on reusing option values and buildout config options reference

* unbroke capitalization

* added another todo

But maybe I should stop doing this because sphinx tells me about
missing references.

* note that paths are whitespace separated.

* fixed some minor wording bugs.

* Added a hidden test to check my understanding of interaction with install-from-cache

* Clarify modes and semantics of extends-cache

* emphasize that in newst mode, we always download configuration

* expanded on logging control

* clarified socket-timeout

* spelling and it's

* simplify user-default example

* warn about unpinning when using develop eggs

* discuss unpinning

* explain why unpinning with empty pin is useful.

* reverted examples to use longer more explict forms

* typos

* Added tests for variables-extending-and-substitutions.rst

Also added more test helpers.