Tidied up imports
Added opening by path

This allows a node to be specified as a dictionary item with the full
path with respect to the root. This mirrors the same functionality in
h5py.
Added a new base class BaseDialog

This is to allow plugin dialogs to inherit from the same class as
import dialogs.
Adds plugin menus

New top-level menu items are added if plugins are found in the
nexpy.plugins directory or the private ~/.nexpy/plugins directory. Also
harmonized a few function names.
Add __init__.py in plugins directory
Fixed coding mistakes

A couple of functions and classes were not properly invoked.
Added the ability to use a node path

This is when using the dictionary form to get and set nodes within a
group, i.e., in __getitem__ and __setitem__.
Merge branch 'plugins'