Add Vedis#append method.

http://vedis.symisc.net/cmd/append.html

```ruby
v = Vedis.new

# If key exists, append a value,
p v[:hoge] = "aaa"        # => "aaa"
p v.append(:hoge, " ccc") # => "aaa ccc"
p v[:hoge]                # => "aaa ccc"

# If key does not exist, set value in key.
p v.append(:fuga, "ccc") # => "ccc"
p v[:fuga]               # => "ccc"

v.close
```