Param fixes

There are two errors preventing some command params from being used properly.

* `GlobalConfig` incorrectly checks for a param named `"size"` which it does not find because it's real name is `"doc-size"`
* `RoadRunner` incorrectly registers both _ratio_ and _ramp_ to param `-r` (lower case r) so only one can work

Fixes I implemented:

* `GlobalConfig` now correctly checks for presence of `doc-size`
* `RoadRunner` now uses ratio at `-R` (capital) and ramp as `-r` (lower) so both options can be used
* `GlobalConfigTest` class added to verify all params can be set properly
* added constants to define param names so typos would not cause any further problem

New feature:

* Allow specifying --data-filename instead of --doc-size which will make the test use the file's contents instead of random bytes