Allows yara to properly include additional yara signatures included through 'include' directives

[ewil]

The example use case is to have an 'all.yara' file that includes multiple other yara signature files. This fails when using the 'source' argument because included files aren't found. We don't want to specify full paths, since the paths differ across the machines our signatures are used on.

yara.compile() doesn't seem to provide a way to to specify the directory context when using the 'source' argument, so the way we got this to work was through this patch.