Can use string keys in constructor

[treeder]

Symbolizes passed in options. All tests pass and created new tests for string options.

Closes #255.

[pietern]

Thanks for submitting this pull request.

Symbolizing all keys in a hash is generally not a good idea, since it can contain anything, and symbols are not GC'd. This means that a large hash with keys unrelated to redis-rb will result in memory being used without purpose. I took a slightly different approach in #259, where only the keys that are used are symbolized.