pop() is a no-op on a tree with one element.

if the last element were to be removed, other methods like eval()
would stop working as expected.
add test_push
settle on a project name, 'fry-blist'.

fry-btree is misleading and easily confused with a binary tree.
fry-bstack doesn't seem like a great fit either.

in the end fry-blist represents a list of Binding objects contained
by an object that implements a basic set of operations.
don't allow an empty list.
add Fry::Binding#first, Fry::Binding#first test
add Fry::BList#tail

returns every element in the list, excluding the first element.
update README.md