Added initial exericse 1.7 code copied from the book.
Exercise 7 solution.

Seems to work okay. Maybe a bit innacurate for very large numbers.
Going to try an alternative I saw online...
Updated 1.7 solution.

This version from http://community.schemewiki.org/?sicp-ex-1.7 is more
accurate. I could've made my original implementation more accurate by
changing the tolerances but this version only requires updating one
value instead of two for increased accuracy.
Added solution for exercise 1.8.

Was able to reuse most of the code from 1.7 for this - just updating
the improve function.