Ability to change jelix context on the fly

[laurentj]

When creating unit tests, or if we want to migrate jelix-scripts to a real jelix application, it should be interesting to have an api which allow us to change the context of Jelix :

• by loading an other configuration file
• by changing all values of JELIX_* constants (so we need to replace these constants by static property in a Jelix class for example)
• by changing values of all $gJ* global variables.
• old values are saved in a container, somewhere, so we could switch between several application.

We need to study consequence of this changes, in session for example.

[laurentj]

In fact, jelix should be reentrant.