Lots of changes

 * Additional documentation on attributes
 * Add a property to AttributeSet, copy_by_default, which subclasses can use to indicate if they should be copied by default
 * Add PosixPermissions.execute, a property that's True if any execute bits are set and False if none of them are; setting it to True sets all execute bits for which the corresponding read bit is set
 * Add a property, devices, to MountPoint, which is a list of instances of a new class, MountDevice
 * Add a property is_mountpoint to BaseFile
 * Implement PosixLocalFileSystem.child()
 * Implement part of PosixLocalMountPoint.unmount()
 * Rename _SSHConnection to SSHFileSystem
 * Move connect and from_transport from SSHFile to SSHFileSystem. I'll be writing replacement wrappers on SSHFile for backward compatibility.
 * Implement SSHFileSystem.child and SSHFileSystem.roots
 * Strip extra leading slashes from SSHFile