Refine argument apis

[hadashiA]

GetArg Other names should consider more explicitly C#-like ?

e.g)

• GetArg(int) -> GetArgumentAt(int)
• GetArgAsString(int) -> GetArgumentAsStringAt(int)

[hadashiA]

mruby's get_args has a check for the number of arguments. Perhaps we should take a similar approach

[hadashiA]

#152