Release v0.3.1 - New SRA exporter, DB importers

This release of isatools implements new ISA to SRA exporters, and 4 data importers into ISA-Tab:

• ISA JSON to SRA export
• Replacement ISA-Tab to SRA export (legacy exporter still included while new exporter becomes stable)
• Importer for MetaboLights to ISA-Tab and ISA JSON
• Importer for MetabolomicsWorkbench to ISA-Tab
• Importer for Biocrates to ISA-Tab
• Importer for European Nucleotide Archive (SRA) to ISA-Tab

For more information on the known issues with this release, please check the documentation at http://isatools.readthedocs.org

This is a minor update that fixes a distribution bug reported in #142.