SPIP commits

This commit introduces a lot of things that make the import really secure :

1. Create a set of tables with the old structure and a prefix not used
   
2. Import data inside
   
3. Update
   
4. copy data to the actual site
   

Of course it's a beginning, a lot of things should be done :

• Make this functionnality possible for older stable versions ([11402] for 1.8)
  
• Verify that the temporary prefix isn't used
  
• Interpolate between different versions for partial imports
  
• Use a reload over timeout during the copy ([11402])
  
• Also import plugins tables (if possible)