Switch backend from Flask to Pyramid

This pull request contains: 

- New pyramid backend
- New installation instructions
- Bug fixes, such as:
    - #38 The map names are now truncated in the Manager tab if they are too long
    - #41 The separation between the editors is clearer 
    - #37 You can now exit and open a different workspace
    - #22 You can now specify mapserver's path
- Some refactoring of the python code. We now have a much better MVC structure. 
- Some refactoring on the javascript side too.