Release 0.9.1

• Fixed #272: Handling appengine special package case.
• Fixed #273: Handle multiple packages in the same directory but handling
  build tags used in those packages.
• Added documentation explaining how import resolution works.
• Fixed #275 and #285: Empty directories as package locations reporting errors.
  Improved the UX and handle the errors.
• Fixed #279: Added Go 1.7 support that no longer has GO15VENDOREXPERIMENT.
• Issue #267: Added os and arch import properties to the documentation.
• Fixed #267: Glide was only walking the import tree based on build flags for
  the current OS and Arch. This is a problem for systems like docker that have
  variation built in.