added wavelegnth for ENVI binary raster-style files.

Hi,
I found your plugin quite useful, and thought it would be useful to add wavelength for hyperspectral applications. I've tested this on Mac OS with ENVI-style binary raster files, and a hdf 5 file I've been developing for our own data distribution (which mirrors the metadata found in ENVI-style rasters). I'm brand new to qgis development, but I'm hoping you might find it useful.

Cheers,
Bryan