refs #1: a start
* bump the version
* provide two methods to use when (re)writing data into an existing file

  * openGroup(): open or create the NeXus/HDF5 group, return the object
  * write_dataset(): write to the NeXus/HDF5 dataset, create it if necessary, return the object

* access documentation for the two API files
* fixes #1 : writes using h5py, drops nexus tree API
* remove tree-only command-line option for now from spec2nexus
update the documentation