PGMReader: pixel data of PPM files is interleaved (issue #1263).
PGMReader: use whitespace instead of lines to delimit entries on file header.

The netpbm file formats has 2 or 3 entries on the header delimited by
whitespace. The current implementation only recognized individual lines.
It is possible that there's no newlines in the entire file which caused
the reader to blow. In adition, this formats also allows for comments
anywhere before the raster data while currently they were only identified
if at the start of a line. Fixed all this by making use of StreamTokenizer.
PGMReader: set metadataComplete to false since comments and stack are ignored.

Because the comments in the file are discarded during read, we are confident
that we there's metadata being discarded. Set metadataComplete to false.
The netpbm file formats support image stacks since July 2000 but only the
first image is actually being read.
PGMReader.java: add ppm and pbm to list of acceptable file extensions.
Merge pull request #1268 from MicronOxford/pgm-fixes

multiple netpbm fixes