Add Rspec
Can access model variables
Store relevant information about model and index in variable
Can multiply variable with scalar
Expression returns 0 as scalar for non tracked variables
Remember all factors of expression
Can concatenate two expressions
Can append variable to expression
Can subtract an expression
Can add variables or expressions to variables
Can subtract values from variable
Unary - for Expression
Unary - for variable
Define * Variable for Fixnum
Can multiply a Float with a variable
Making Numeric Extensions DRY
Updated Readme
Can add a constant factor to an expression
Can add Expression to Numeric
Can subtract Numeric from Expression