Salesforce Date compare

When comparing date fields (in unix time) with the formula `NOW()` (which returns a SaleforceDate object), the parser will currently throw an `UnsupportedTypeError`: `Function larger(object, number) not supported`.

This PR will allow salesforce date math with the following comparison operators: 

`+`
`-`
`==`
`<>` and `!=`
`<` and `<=` and `>` and `>=`