Tests in test directory weren't really that useful.  Removed them.
Changed code to correctly assemble rules from parent directories.  Before this commit, had a problem where if a rule in a parent directory had some directory structure, would just ignore the directory structure.  Now, elide the parts of the directory structure that match the subdirectory from which we execute the command.
Previous version of code misinterpretted grep's "exclude" command.
Previous interpretation was that it could match directory structures.
But what it actually does is that it will exclude a file only if the
filename (excluding its directory) matches the argument to exclude.

Re-formatted to use find, relying on find's -exec function to execute
a single grep per file that doesn't match the patterns excluded in
.sgrep files.

As an example of what would execute if one of your .sgrep files had
the rule "*pyc" and you sgrep-ed for "something"

find . -path "./*pyc" -prune -o -exec grep something -H {} ;

Ie, for each file from this directory that does not match *pyc,
execute a grep on it.
Improved the README so that it is actually kind of sort of worthwhile.