Add new fn find-files*; redefine find-files in terms of find-files*

`find-files*` is like `find-files` but allows the use the use of a
predicate function to select files instead of a regular expression.
`find-files` has been redefined in terms of `find-files*`.

Add test for both functions.
Use filter instead of for
Merge pull request #53 from noprompt/master

Add new function find-files*