Remove the legacy 'special' distribution, because it was unscientific
and hard to explain.
Pareto and Zipfian distributions provide more clearly defined
alternatives.
Adds a new random generator of variable-length strings:
```lua
s = sysbench.rand.varstring(10, 100)
```
"s" will be a string containing ASCII characters from '0' to 'z',
inclusive. It will be a length between 10 and 100 characters long.
Addresses GH Issue #17
Use ideas and code from https://github.com/preshing/RandomSequence to
generate a sequence of unique 32-bit numbers. Make the code thread-safe
as the corresponding Lua function in sysbench might be used in a
concurrent context.
