Merging of pmishev and falahati changes

Translated German comments to English
Handle 'null' values in formula as well
Made all unary operators on NULL to return again NULL
Return NULL when empty formula is given

Now supports "Less than", "Greater than", "Less than or Equal", "Greater than or Equal", "Equal" and "Not Equal" as well as "AND", "OR" and "XOR", ( > , < , >= , <= , = , <> , >< , & , | )