bootstrap test environment with minitest & minitest/unit.

minitest/autorun.rb looks like a costly operation in a rubygems and/or
bundler environment. there is no need to try to require rubygems, call
Kernel#gem, etc, so skip it.
update .gitignore
add Fry::Binding and associated tests.
README.md: mention pry in repl list.
add Fry::BasicObject::ArgumentError.
Fry::BStack update.

Kernel.raise on an empty stack in #initialize.
HOME is equal to ~.
push a wrapped Binding in Fry::BindingStack.push().