Lua => LuaScript.

"LuaScript" is a better name for this class, as it represents a specific lua script, and not the lua language as a whole.

While I was at it, I moved the USING_LEGACY_REDIS_VERSION conditional out of the method and into the class body -- might as well evaulate that branch once at load time rather than once per script call.
Implement a lua script cache.

It's unnecessary (and slows things down) to load the same script into the same
redis server multiple times in the same process.
Merge pull request #79 from seomoz/lua_caching

Lua caching