give virtualenv directory a more generic name, "venv"

pros:

* Installation instructions uses the same name
* The old name assumes user would be using Python 2.6
Merge pull request #752 from tshepang/master

give virtualenv directory a more generic name
Small fix for RST misfire.
fix #746: explain why a dev package is installed
typos
improve wording regarding pyvenv
eliminate repeated "the" words
eliminate other repeated words
Merge branch 'master' of github.com:Pylons/pyramid
Merge branch 'issue746' of git://github.com/tshepang/pyramid into tshepang-issue746
fix rationale and typo
Merge branch 'tshepang-issue746'
Merge pull request #749 from irskep/patch-1

Mention ZopeTransactionExtension and pyramid_tm in SQLAlchemy scaffold tutorial
Merge branch 'pyvenv' of git://github.com/tshepang/pyramid into tshepang-pyvenv
reflow, typo
Merge branch 'tshepang-pyvenv'
Merge branch 'master' of github.com:Pylons/pyramid
Merge pull request #760 from tshepang/typos

lots of typos
Merge branch 'master' of github.com:Pylons/pyramid
Reduce duplicated installation instructions from the tutorials

* This also removes the mention of running 'source bin/activate' (see #759).