Allow @ in URLs

Without this patch, Gitian-Builder fails with an _unsanitary string_ error in `gbuild` when pulling down source code stored on BitBucket.org. The same change is needed for `gsign` and `gverify` when run against signature repositories also on BitBucket.

`gsign` and `gverify` were also updated to allow the full range of characters accepted by `gbuild` as previously fixed in 8ad9e0dfaf4fff357e972c80960cd416c226ab4d.