Add symlink item

CC #13
Add tests for symlinks

CC #13
Add documentation for symlinks

CC #13
A symlink certainly needs a target

CC #13
Merge branch 'symlink-item'

Closes #13
update list of target requirements
keep item template short
add method to validate an item's name
add method to tinker with user-specified attributes
Merge remote-tracking branch 'github/master' into users-item