Mention ZopeTxnExt and pyramid_tm in tutorial

If this page is found via web search while trying to configure
SQLAlchemy from scratch without a scaffold, it is helpful to
mention pyramid_tm as a requirement somewhere. Otherwise, a
hapless user such as your humble author might spend lots of
time wondering why his sessions aren't getting committed after
his requests despite his best efforts to properly configure
SQLAlchemy and ZopeTransactionExtension.
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' into activate