Add pretty printing option and fix segfault on parsing empty strings

Use `JSON.generate(json, {:pretty_print => true, :indent_with => "\t"})` to generate pretty printed json indented with tab character.

Default options are `{:pretty_print => false, :indent_with => 2}` (`:indent_with` can be both string and Fixnum, where Fixnum specifies how many spaces are used).

`JSON.generate("")` used to cause segfault. Now returns `mrb_nil_value()`

Example:
```ruby
test = {
  "Image" => {
    "Width" => 800,
    "Height" => 600,
    "Title" => "View from 15th Floor",
    "Thumbnail" => {
      "Url" => "http://www.example.com/image/481989943",
      "Height" => 125,
      "Width" => "100"
    },
    "IDs" => [116, 943, 234, 38793]
  }
}
puts JSON.generate(test)
puts JSON.generate(test, {:pretty_print => true, :indent_with => 2})

# Output
# {"Image":{"Width":800,"Height":600,"Title":"View from 15th Floor","Thumbnail":{"Url":"http://www.example.com/image/481989943","Height":125,"Width":"100"},"IDs":[116,943,234,38793]}}
# {
#   "Image": {
#     "Width": 800,
#     "Height": 600,
#     "Title": "View from 15th Floor",
#     "Thumbnail": {
#       "Url": "http://www.example.com/image/481989943",
#       "Height": 125,
#       "Width": "100"
#     },
#     "IDs": [
#       116,
#       943,
#       234,
#       38793
#     ]
#   }
# }
```