Try to validate addon grossly before doing anything

[myselfhimself]

for blender-addon-tester 0.1, when piping a non-plugin path (eg. pictures directory), blender gets downloaded first, then blender detects that it's not a plugin..
We could detect grossly that there are files in it (compressed or uncompressed dir), with one file exposing one type of comment or methods, with a proper directories layout.

Note that such an archive validator could be imitated also to validate that: any plugin is a plugin according to a spec (could be used to check hundreds of them), any blender release is a supported blender release (eg. tar.bz2, tar.gz, tar.xz, dmg, *lender.app with executable in Contents/MacOSX etc..).