Add hashbang to build_book.rb
Use require_relative for ruby >1.9 compatibility
Rely on PATH for kindlegen invocation
Don't use deprecated -rebuild kindlegen flag

kindlegen 2.9 does not support the -rebuild flag. This patch removes the
use of that flag.

This patch also (I know, I know) adds some primitive error handling of
the kindlegen invocation.
Add hashbang to build_book.rb

Signed-off-by: Anton Lofgren <alofgren@op5.com>
Add help text for a couple of variants of help flags
Merge pull request #10 from catharsis/sicp-kindle-patches

Sicp kindle compat. & usability patches