Updated Clojure version and project URL.
Removed merge-agents.

After the elimination of agent-types, the only way that merge-agents
differed from an ordinary merge was in the concatenation of
draw-functions. But the ability to combine draw-functions wasn't
actually being used anyway, so I have reverted to ordinary merge for
simplicity.
Changed :draw-functions to :draw-function.

Previously each agent could have a sequence of draw functions, all of
which would be called. Since this wasn't being used, I made the
simplification that each agent would have just one draw-function. So
now instead of having :draw-functions an agent has a :draw-function,
the value of which should be a function that takes the agent as an
argument.
Started documentation improvements.

Still in progress...
Fixed rotation proposal handling

Removed :from-velocity special case since the same effect can be
achieved with a proposal for :rotation (direction->rotation (:velocity
p)). Also now properly handle an absence of a proposal for a rotation;
it will now not rotate at all, whereas previously it assumed a proposal
for rotation 0.
Improved documentation and misc. cleanup.

Enhanced readme.md, eliminated unused utilities, moved a few
definitions to more natural places, and added documentation strings to
function definitions.
Version bump.

To 0.1.3.