serializing non-string objects

This will prevent type errors when trying to save any Ruby objects that aren't a String. 
It will serialise them using Marshall. 

```ruby
require 'lmdb' ; env = LMDB.new('.') ; maindb = env.database
=> #<LMDB::Database:0x007f87678f1ba0>
maindb['foo'] = 1
# TypeError: no implicit conversion of Fixnum into String
maindb['foo']
=> nil
maindb['foo'] = :bar
# TypeError: no implicit conversion of Symbol into String 
```

Thoughts?