Fix Propel TreeBuilder

Fix Propel TreeBuilder to correctly normalize xml configuration file.
This commit fixes #682.

Be careful: starting from now, we introduce a new (more standard) way to describe arrays in xml. See the following examples:

**BEFORE**
```xml
<settings>
  <charset>utf-8</charset>
  <queries>
    <query>select * from table1;</query>
    <query>select * from table2;</query>
  </queries>
</settings>
```

**NOW**
```xml
<settings>
  <charset>utf-8</charset>
  <query>select * from table1;</query>
  <query>select * from table2;</query>
</settings>
```
I'm updating the documentation in this PR https://github.com/propelorm/propelorm.github.com/pull/296