GNUmakefile: flexibility in the do-check target

allows overriding the mocha command, list of test files.
preparations for app.configure

there's a partial implementation which just checks the arguments
so far, and the tests don't assert anything beyond that.

the implementation currently recognizes only RAM:, disk:, swap:
(the easy case); i'm leaving the rest for later.

also, it is a copy/paste of app.add to keep the diff simple,
i'll DRY it up in the next commit.
app.configure shares code with app.add
package.json was missing some files from manifest
test the low-level API .session w/ default rpcimpl

turned out all the tests so far were providing their
own rpcimpl, and the default case had a bug of course:
lib/lo/index:session was expecting to find rpc exported
from lib/lo/{admin,user}, when in fact it's exposed by
lib/lo/common.  (rpcimpl or lo.rpc) was undefined
instead of a function.
fix the bug described in parent
lo/common#api uses Error instances, not strings

this placates Mocha and is better behavior anyway.
coffee-script dep bumped up to ~1.4
mocha dep bumped up to ~1.8
chai dep bumped up to ~1.5