Added possibility to apply "LIMIT 0" in PostgreSQL adapter

This is possible in Postgres and may be useful. "OFFSET 0" seems useless but I think it should be added for consistency.
removed un-used PdoConnection class
added new transaction API. added initial tests.
Use fenced code blocks instead of highlight tags

Rely on Markdown's fenced code blocks instead of Jekyll Liquid tag
Update informative files

Update files for contributors and/or readers such as the index file
and the contribution file
Update the configuration for fenced code blocks

The fenced code block syntax is provided by Redcarpet so the Jekyll's
configuration should be updated to make them work and also ensure the
site looks the same at some levels (e.g.: tables)
Fix various typos and deprecated code in guides

Update the typos in most guides and also update or remove some
deprecated code.
Merge pull request #316 from robin850/master

Documentation update
Fixed test:prepare params for Postgresql

I don't know if anyone has run the tests, but I'm unable to pass the preparations. The tables are duplicated in each schema, they have multiple primary keys which are not included in foreign keys... Even after fixing the schemas, I receive fatal:
``
Propel\Generator\Exception\EngineException: Table "book" declared twice
``
port PR #704 from Propel 1.6
port PR #704 from Propel 1.6
Merge pull request #401 from staabm/transapi

Transaction API
Merge pull request #416 from auss/patch-2

port PR #704 from Propel 1.6
Merge pull request #415 from auss/patch-1

Fixed test:prepare params for Postgresql
fix failed jekyl compilation due to highlight tags
Merge pull request #419 from lunika/master

 	fix failed jekyl compilation due to highlight tags
change default locale from en_EN to en_US
Merge pull request #420 from lunika/default_locale

change default locale from en_EN to en_US
allow to use reference-only on external-schema tag
don't generate sql if skipSql equal true on parent table