Import and export features for tree sequence

Feature release adding new import and export features to the API
and CLI.

• New TreeSequence.write_records and TreeSequence.write_mutations
  methods to serialise a tree sequence in a human readable text format.
• New msprime.load_txt() method that parses the above formats, and
  allows msprime to read in data from external sources.
• New TreeSequence.write_vcf method to write mutation information
  in VCF format.
• Miscellaneous documentation fixes.