Compose app images without yaml, support Hyperkit and Intel HAXM

• QEMU: Added HAXM acceleration for Mac OSX
• Supports running OSv on Docker's Hyperkit on Mac
• Supports rapidly composing apps without a need of explicit YAML manifest files by using a default manifest and new --require option added to package compose
• Enhanced following commands:
  • package describe: added option "-c" to list the content of a specified package
  • package compose: add loader_image option to allow specifying arbitrary OSv kernel
• Made github unit tests more reliable and way faster by using mock
• Revised documentation