Add mem backend

[5.2.0] - 2019-07-16

Added

• In-Memory backend vfs implementation
• Included the in-memory backend to the list of registered backends used in backend_integration_test
• Checking for existence at the start of various File functions (size, seek, etc) and returning and error if the file does not exist on the OS backend
• Tests in backend_integration_test that ensure that operations done on non-existent files throw an error
• mem.md in vfs/docs, updated link to it in the README.md

Fixed

• Relative path validation in utils.go now disallows empty names

Changed

• utils_test.go now expects an empty string NOT to validate
• updated README to include "Touch()" definition under the File interface rather than the Location interface
• updated README to exclude "in-memory-backend" from the list of ideas