Improvements on field name validations:

- field name should be a word character. Only one of [a-zA-Z_0-9]
  characters are valid.

- the later and the reserved words validations are checked only when
  saving schemas, not when creating the layer, which is supported by
  gvSIG (it can read and write, for example, shapefiles with special
  characters - so ArcGIS does, but not QGIS). If we want to change
  that behaviour, we should assess how it migth affect to other parts
  of the application.