Nexpy

- Fixed import error in data dialogs

- Made 'spec2nexus' an optional import in setup.py.
- Used RotatingFileHandler to rotate log files when they are larger then 10kb.
- Fixed a bug when plotting 2D data with log intensity scales. This could prevent axis switching in 3D or higher dimensional arrays, when all the data in the plotted top layer is less than or equal to zero.

This version contains a few significant features.
- An improved method for synchronizing the console and tree
- A new script editor with syntax coloring for editing and running scripts in the console namespace
- A new plugin menu system for customizing NeXpy with domain-specific menus
- Logging of tree activities using the Python logging module

- Adds backup and restore functions to the NXroot group, allowing NeXus files to be saved to disk and recovered if necessary. By default, the backup is saved in the current directory with a randomized string added to the original filename, so that it is easy to clean up unneeded backups.
- Improves handling of missing axes.
- Fixes a bug in setting NXlinks to be the signal.
- Corrects an inconsistency in the way that the `axes` attributes are set by either NXdata group creation or using the `nxaxes` property.
- Improves output of string arrays when printing NeXus trees.
- Improves Python 3 compatibility.

- Improves the handling of fixed-length strings that contain non-ASCII characters. These are now stored as byte strings within the NXfield.
- In Python 2, the NXfield returns an encoded byte string for printing, rather than a Unicode string. This prevents some Unicode encoding errors.

- New NXfield properties have been defined for HDF5 parameters, including compression, chunk sizes, fill values, and maximum shapes. These can be set at any time before the data is saved to an HDF5 file or before slabs are stored in core memory files (for large datasets that are filled as slabs after creation).
- Fixes a bug in Python 3 when storing lists of strings. These were converted to Numpy arrays of kind 'U', which h5py was unable to convert. Now, they are automatically converted to Numpy arrays of variable-length strings, using the special h5py object datatype. If fixed-length strings are required, a dtype of 'S' (or 'Sn' where `n` is the required length) should be used when creating the NXfield.
- Provides better parsing of input parameters when creating an NXfield, _e.g._, the `maxshape` attribute is checked to ensure it is compatible with the NXfield shape.