Read/Write/Execute access by specific user

Issue https://github.com/mizzy/serverspec/issues/96 in the original project.

I've changed be_readable, be_writable and be_executable to suppor the .by_user chain, so that we can test if a given user has explicit read/write/execute access for a given file/directory.

If this is accepted I will implement also for checking access for a specific group, following a slightly different approach however.