Adding interface for NumeralsConversion class

Rechecked spec and realised we need to implement an interface. Removed
constructor which set the instances $conversionValue to the passed in
value since we'll now be providing that as a parameter to the conversion
function calls, as per the interface.
Updated convert view module

Renamed sValue to value since we don't know if it's a string or number
until later. Added check to ensure value is between 1 and 3999 if it's a
number.