Do not process VV as it is invalid

I normally would throw an exception here, but it seems that that isn't
preferred in Objective-C/Swift.  For now I will return 0, and just
modify this accordingly.
Moved roman numeral string validation as an extension to String
Do not process LL as it is invalid
Do not process DD as it is invalid
Moved out invalid roman numeral strings to it's own constant

I really wanted to make this a field level constant, but it looks like I
need to define getters/setters as this is an extension?
Do not process IIII as it is invalid
Do not process XXXX as it is invalid
Do not process CCCC as it is invalid
Do not process MMMM as it is invalid
We should not be able to deduct V
We should not be able to deduct L
We should not be able to deduct D
We should not be able to deduct I from L onwards
We should not be able to deduct X from D onwards