Fixed project_tree, now only looking inside src/

If the project root folder contains for example a build/ or a dist/
folder, which may contain some of the sub folders which project_tree is
looking for, then the result may not bee the expected.

This error is fixed by forcing project_tree to search for sub folders
inside the src/ folder only.
Minor bug fixes of original code.

The super call in __init__ of the MaltegoException class was given the
Super class and not the sub class.

The appendelement and removeelement of the _Entity class was trying to
add and remove elements from the underlying fields and labels variables
as if they were lists. However they were actually dictionaries.
Merge pull request #32 from reenberg/fix/project_tree

Fixed project_tree, now only looking inside src/
Merge pull request #34 from reenberg/fix/maltego-message

Minor bug fixes of original code.