Add JSON Object Value Generator

Add `json_obj_valgen` to the Yokozuna BB driver. I used this to test
indexing "large" JSON objects while investigating #358.  The config
line to use this value generator looks like so:

```
{value_generator, {function, yz_driver, json_obj_valgen,
[<NumFields>]}}.
```

This will generate a static JSON object (i.e. generated once at start
up) which contains `<NumFields>` integer fields.  All workers will
index the identical object.  Here is an example config from one of my
testing runs:

```
{mode, max}.
{concurrent, 32}.
{driver, yz_driver}.
{code_paths, ["/root/work/yz-bb/misc/bench"]}.
{secure, false}.
{bucket, {<<"data">>, <<"largeobj">>}}.
{index, <<"largeobj">>}.
{pb_conns, [{"10.0.1.201", 8087}]}.
{http_conns, []}.
{duration, infinity}.
{key_generator, {to_binstr, "~B", {partitioned_sequential_int, 0,
1000}}}.
{value_generator, {function, yz_driver, json_obj_valgen, [4000]}}.
{operations, [{load_pb, 1}]}.
```

This uses 32 workers to write the same JSON object containing 4000
integer fields over the key space 0 to 999.
Merge pull request #368 from basho/rz/bb-json-obj

Add JSON Object Value Generator

Reviewed-by: lordnull