Change the eval- and expand- functions to take strings

This provides a better interface than the thunks, which added an
extra layer of indirection for no benefit.
Patch up sps-expand and update docstrings

Re-order the actions in sps-expand's callback so that expansion
buffer's major mode is activated after the code is inserted. In
the previous order JS2 wouldn't fontify the buffer for some time
after the expansion was displayed (moving point within the buffer
also seemed to kick it off). I wasn't able to identify a function
that forced fontification - js2-mode-apply-deferred-properties
sounded promising but didn't do the trick.
Remove now-unused sps-chop- functions
Remove functions for automatically enabling

Instead, simply document how to do it. The feature wasn't really
an improvement over doing it yourself, and to make it more
automatic wouldn't be a good fit (e.g. as far as I know there's
no standard Parenscript file extension).
Change default expansion mode to javascript-mode

Using js2-mode seems to trigger a bug causing unexpected code
evaluation when performing expansions and even occasional Emacs
freezes.
Add a README