Update the whole documentation

Hello,

Sorry I'm opening another WIP but I have some questions. I hope this time I will be a little bit faster to update the code. Here's the roadmap:

* [x] **Update the deprecated commands** : According to the `UPDATE.md`, the commands use in the documentation have all been updated (except those which haven't been yet refactored). Also change all occurence of propel-gen with the propel script.

* [ ] **Update all commands' output**

* **Update deprecated methods and objects** :
    * [x] `getConnection` -> `getReadConnection` / `getWriteConnection`
    * [x] `Persistent` -> `ActiveRecordInterface`
    * [x] `BaseObject`
    * [x] `containsColumn` -> `hasColumn`
    * [x] `addParameter(array)` -> `setParameter(name, value)`

* [x] **Remove code related to PHP < 5.4** : Remove all mentions to options/code related to PHP 5.2 or 5.3 (e.g. magic_quotes).

* [ ] **Write a guide to upgrade from Propel 1.x to 2.0** : Write a guide widely based on the `UPDATE.md` file and with the new stuff (e.g. new folder's structure).

* [x] **Update all relative paths that point to Propel's installation directory** : Let's rely on Composer instead.

* [x] **Update all links pointing to Propel 1.x repository**

* [x] **Update references to generated folders** for instance, the generated sql is no longer in `build/sql` but in `generated-sql`.

* **Remove deprecated commands**
  * [x] dbd2propel
  * [x] datasql
  * [x] datadump

* [ ] **Check the whole documentation for deprecated mentions/code/features/anything**.

I've also made few improvements:

* Update the contribution guideline:
  * Update the workflow since the [repository's design change](https://github.com/blog/1529-repository-next)
  * Move the section about contributing from the README to the `contribute.markdown` file.
  * Enhance the fixing checkstyle mention to only fix files related to the PR.
* Update the [svn checkout url](https://github.com/blog/1438-improved-svn-here-to-stay-old-svn-going-away) 

Have a nice day