Fixed Firefox autoload fails to find .xpi file.

Removes the version suffix from the build to always build just "dreditor.xpi".  Adding a version suffix is the responsibility of a build/release archiver post-process.
Fixed readability/coding-style of build:safari grunt task.
Adjusted all extension builds to use just dreditor.* as artifact filename.

Artifact filenames may clash, so use a vendor-specific subdirectory in ./release for each extension.
Merge pull request #125 from sun/ff-build-autoload

Fixed Firefox autoload fails to find .xpi file.