Release 0.3.1

• Exit with a non-zero exit code when errors occur
  (contributed by C.A.M. Gerlach)
• Type check the working directory if no command-line arguments
  are given (contributed by C.A.M. Gerlach)

Release 0.3.0

• Type check calls on Unions properly
• Add pyanalyze executable
• Add --enable-all and --disable-all flags
  (contributed by C.A.M. Gerlach)
• Bug fixes