Introduced a PgAdmin class inherited from the Flask, which looks for

submodules inherited from the PgAdminModule instead of regular
Blueprint. This allows us to load the module automatically from the
under the pgadmin directory, and will work to extend the pgAdmin
extension module.

PgAdminModule is inherited from the Blueprint, and bring several
methods:
-  get_own_stylesheets, which returns the stylesheets used by the module
   (excluding its submodules stylesheets)
- get_own_javascripts
- menu_items, which returns a dictionray mapping the old hook names
  (context_items etc) to a list of MenuItem instances

For more specialized modules (as for now, any module that should be part
of the browser tree construction), one can define an abstract base class
defining additional methods.

For example, the BrowserPluginModule abstract base class defines the
following methods:
- jssnippets
- csssnipeets
- node_type
- get_nodes

web/pgadmin/utils/menu.py

@@ -0,0 +1,6 @@
+from collections import namedtuple
+
+class MenuItem(object):
+
+    def __init__(self, **kwargs):
+        self.__dict__.update(**kwargs)