RStoolbox 0.1.8

This is primarily a maintenance release.

New:

• spectralIndices() can now apply a mask internally, e.g. to exclude cloud pixels. New arguments are:
  maskLayer and maskValue (suggested by Andrea Hess).
• added spectral index GNDWI

Fixes:

• update readEE() to deal with new EarthExplorer export columns (reported by Christian Bauer)