Features for TreePath

I implemented some features for TreePath.

- Annotation Attribute "appendId" for @Gedmo\TreePath allows you to set manually if the id should be appended to the path or not.
-  Annotation Attribute "startsWithSeparator" for @Gedmo\TreePath allows you to set manually if path should start with the separator or not.
-  Annotation Attribute "endsWithSeparator" for @Gedmo\TreePath allows you to set manually if path should end with the separator or not.
-  Annotation @Gedmo\TreePathHash allows you to define a field that is automatically filled with the md5 hash of the path. This field is neccessary if you want to set a unique constraint. Some Databases dosn't allow that a unique contraint is longer than like 200 characters (which a path definitly could get)

I did NOT implemented the features for XML and YAML, i have no experience with these one.

Relevant issue: https://github.com/l3pp4rd/DoctrineExtensions/issues/526