make_cap now takes a function.

This function specifies v (the angle down from the primary axis) as a
function of u (the angle around the axis). Changing this function lets
you do cool things like make stars.
Added unitary arg to hrr constructor.
Can use Bootstrapper as context manager now.

Added a module level add_data function which will add data to the
closest enclosing Bootstrapper-managed context.
Added filestrap decorator.

Easy persistent memoization using Bootstrap object.

Currently unsure about the memoization arguments. Should the user,
at decoration time, specify all the arguments that could possibly
be memoized, or all the arguments that ARE memozied? That is, when the
function is called, do we require that all memoized variables be
provided to the function? The alternative is to just memoize based on
presented arguments that have also been designated as memoizable.
Prefix can contain directory.

Directory will be created if it doesn't already exist.
Reworked memoized_args in filestrap.

memoized args is a list of names of variables that are relevant to
memoization if they are provided, but not all arguments in memoized_args
have to be provided.

Also, will look into default values to get values for memoized_args.
Filestrap now gets from default values properly.

For memoized argument names.
file_helpers.make_filename now sorts config dict.

Alphabetically using the keys.
Pep 8.