Adding Script Manager

Adding a class to manage pre-loading & evaluating Lua scripts on Redis.

Example:
```
Redistat::ScriptManager.load_scripts('./lib/scripts')
```
If lib/scripts contains a file "sum.lua" then you can do: 
```
Redistat::ScriptManager.sum
```
to run the script.  

Additionally, the method takes arguments for any keys or args that need to be passed into the script:
```
keys = []
argv = []
keys << 'foo'
argv << 'bar'
Redistat::ScriptManager.sum(keys, argv)
```

@jdoconnor