Add snippet of yang file into comments of generated classes

When we generate a class we should add some simple meta data to the comments to help people understand its purpose.

For example we should add, in addition to the "description" field in the yang file, the following information to the java comments:

1) Add a snippet of yang that this class represents
2) Add the type this file represents (i.e. notification, grouping, container etc)
3) Add java doc links to related generated classes
4) Add path name to yang file
5) Add java doc links to the gitweb file that the file was generated from (this would encompass part of #4 too so maybe we could skip that).