Merge pull request #10 from Deric-W/abc

add predefined variables

Merge pull request #3 from Deric-W/cleanup

mark multiple things as final

fix documentation build

increment patch version

correct README statement about recursive terms

BetaNormalisingVisitor can not produce cycles
because it does not modify terms, preventing them
from referencing terms available after their construction.

add terms.combinators

improve CollisionError string representations

improve BetaNormalisingVisitor and Substitutions

This commit modifies BetaNormalisingVisitor to yield alpha conversions,
which required refactoring the Visitors used for substitution

add method chaining for term construction

fix license display, again