reading sprite config with project level

[adyliu]

Add 'Project-level' configuration support.

http://glue.readthedocs.org/en/latest/files.html#introduction

sprites
    ├── actions
    │   ├── add.png
    │   ├── remove.png
    │   └── sprite.conf
    └── icons
    │   ├── comment.png
    │   ├── new.png
    │   └── rss.png
    └── sprite.conf

The arguments in the sprite-level configuration file can overwrite the value of 'sprite.conf' in the project folder.

The glue DO NOT read the arguments in the 'Project-level' file after v0.9 and that's different with v0.3 before.

So I add this future back to the latest version.

[jorgebastida]

I've changed your patch a little bit and wrote some regression tests. Merged into master. Thanks!

[jorgebastida]

released as part of glue 0.9.4