Fixed issue #24

There were changes made to parse::Environment and parse::DefaultEnvironment to store the recursion level. This may cause issues with any other programs which depend on this crate. Now, in order to initialize the parse::DefaultEnvironment struct, one must use parse::DefaultEnvironment::new(). There was also a change to the parse::Environment trait. It now requires the functions add_recursion_level(&self) subtract_recursion_level(&mut self) and get_recursion_level(&self) Finally, a new variant was added in the error::CalcError enum. This variant is the error::CalcError::RecursionLimitReached