[ruby/pstore] Emphasize keys instead of roots, values instead of objects (https://github.com/ruby/pstore/pull/7)

Modifies RDoc to Emphasize keys instead of roots, values instead of objects.

Code:

    Renames method #root? to #key? and method #roots to #keys.
    Aliases method #key as #root and method #keys as #roots.
    Adds testing for all four methods.

https://github.com/ruby/pstore/commit/4436ea0891