REP-0140: initial copy of REP-0127
REP-0140: initial review draft
REP-0140: correct errors and omissions found by reviewers
REP-0140: rename <devel_depend> to <build_export_depend>
REP-0140: fix heading formatting error
REP-0140: clarify use and definitions of <export> sub-tags
REP-0140: try to explain <conflicts> and <replaces> more clearly
REP-0140: allow <test_depend> to co-exist with other dependencies
rep-0140: add <buildtool_export_depend> and <doc_depend>
whitespace cleanup
use capitalized words in titles
fixup example package.xml, important changes

The important change is that catkin must be a
buildtool_depend and no a depend
adjustments to wording, extended intro

intro now include references to doc_depend and
changes to test_depend semantics
reorder list of depend tags

I did this mostly because exec_depend should come
after plain depend, since depend is a supper set
of exec_depend.
fix typo in description of doc_depend
rep-0144: fix section heading underscores