Sanitize version names when creating egg paths.

Branches that contain invalid filesystem characters (or special
filesystem characters) would have them passed as such.  This patch
replaces non alphanumeric + [-_] characters with underscores.

As an example, a spider deployed from git branch "feature/x_y" would
attempt to store the egg in a nonexistent directory called "feature"
and fail.
Merge pull request #72 from andrewbaxter/sanitize_eggpaths

Sanitize version names when creating egg paths.