quick hack to allow module names that aren't uppercase/have underscore/e...

right now obuild is a little picky in the kind of names it accepts for modules in `init` mode. this patch tweaks obuild to acept module names that

- contain underscores
- lowercase module name
- have `.ml` extension

One caveat is that when the obuild file is written the case of the module name is not preserved (still better than not being written at all though)