Support assembly coverage formats

Assembly coverage information can be stored in well-known formats: pileup, TDF, wig: http://genomeview.org/manual/Preparing_value_data (old link: http://genomeview.org/content/preparing-pileup)
We have to support such formats for import/export. There should be a possibility to work with zipped files. An optional threshold value should be introduced, regions that don't match this value should be skipped.

Information to export (if the output file format supports it):
coverage (quantity reads per position)
quantity of every symbol per position (optional)

A workflow designer worker and a sample is to be added.