0.2.0

CHANGES:

• Documentation fixes and restructuring (@polytypic)
• Scheduler friendly waitpid, wait, and system in Picos_stdio.Unix for
  platforms other than Windows (@polytypic)
• Added Picos_select.configure to allow, and sometimes require, configuring
  Picos_select for co-operation with libraries that also deal with signals
  (@polytypic)
• Moved Picos_tls into Picos_thread.TLS (@polytypic)
• Enhanced sleep and sleepf in Picos_stdio.Unix to block in a scheduler
  friendly manner (@polytypic)