Allow basho_bench to pre-compute and pre-load Riak vnode data dirs.

Currently this only works with the eleveldb driver.  For maximum
utility, it should be extended to work with bitcask and other
Riak backends.
Add comments to examples/shortcut.config
Add support for Bitcask

Test method:

* Use the `examples/shortcut.config` file as shown to pre-populate 50K keys.
  Change the `code_paths` and `shortcut_data_dir` to reflect your
  environment.
* Run `./basho_bench ./examples/shortcut.config`
* Over on a Riak console, use:
  * {ok, C} = riak:local_client().
  * [ok] = lists:usort([catch element(1, C:get(<<"b1">>, <<X:32/native>>)) || X <- lists:seq(0,49999)]).
Add support for experimental hanoidb storage manager
Refactor preflist calculation, clean up debugging cruft
verify seq_int resume value is not out of bounds
Updated http_raw driver to allow appending custom headers in the .config file, which is useful for things like HTTP Auth.
Merge of branch  slf-bulk-insert-shortcut2
Merge pull request #68 from joecaswell/jhc-resumecheck

verify seq_int resume value is not out of bounds
Merge pull request #69 from nathanaschbacher/master

Updated http_raw driver to allow appending custom headers.