move the URM length definition to the initial definition of an URM.
Explicitly set out the second definition of URM which replaces the first
bold the definitions in URM definition
computability - add defs for concatenation of URMs and working space
tweak wording
remove require.js reference. there is no such thing.
add example for computing the composition of computable functions (unary functions to begin)
Add example for composition with a binary function