v0.4.0

• Modifed file schema: v2
• Support for nested Cooler "trees" at any depth in an HDF5 hierarchy
• cooler.io: tools for cooler creation
  • cooler.io.write is a generic writer that takes input from a contact reader
  • Contact readers can take a variety of input (aggregate from a contact list, load from an existing matrix, etc.)
• Added new scripts for contact aggregation, loading, dumping and balancing
• API changes
  • Table views are a bit more intuitive: selecting field names on table view objects returns a new view on the subset of columns.
  • New API function: cooler.annotate for doing joins
• Documentation updated