Version Control With Subversion Source

Finish chapter 4 edits suggested by offby1 in issue 62.

* src/en/book/ch04-branching-and-merging.xml: lots of small wording
tweaks, and one new <tip>

s/schema/scheme/ when talking about URLs. Fix some other uses of 'schema' to
use more common terminology.

Note that 'svnadmin verify' fails as soon as a problem is found.

Tweak an xref to point to a more relevant related section.

Tweaks made or suggested by Eric Hanchrow.

Make 'backdate' into a <firstterm>, with a concise explanation.

Tweaks from Eric Hanchrow, plus some new stuff on language bindings.

Commit a bunch of tweaks from Eric Hanchrow's review of chapter 3.

Add a note about scheme vs. schema to HACKING.

* book/ch07-customizing-svn.xml
Document the http-proxy-exceptions runtime configuration option.
Oversighted noted by Neil Mayhew <neil_mayhew@...>.