Support authenticated HTTP URLs

Software `source` method now supports the `authorization` option.
This allows the user to supply an HTTP Authorization header which will
be passed to the source URL.

For the common case of basic authentication, that looks like this:

```
require 'base64'

username = 'XXX'
password = 'YYY'
credentials = Base64.encode64("#{username}:#{password}")

version 'the_version' do
  source url: 'https://example.com/path/to/the/file',
         authorization: "Basic #{credentials}",
         ...
end
```

lib/omnibus/fetchers/net_fetcher.rb

@@ -169,6 +169,7 @@ def download
 
       # Set the cookie if one was given
       options["Cookie"] = source[:cookie] if source[:cookie]
+      options["Authorization"] = source[:authorization] if source[:authorization]
 
       download_file!(download_url, downloaded_file, options)
     end

lib/omnibus/software.rb

@@ -263,6 +263,8 @@ def dependency(val)
     #
     # @option val [String] :cookie (nil)
     #   a cookie to set
+    # @option val [String] :authorization (nil)
+    #   an authorization header to set
     # @option val [String] :warning (nil)
     #   a warning message to print when downloading
     # @option val [Symbol] :extract (nil)
@@ -294,7 +296,7 @@ def source(val = NULL)
         extra_keys = val.keys - [
           :git, :file, :path, :url, # fetcher types
           :md5, :sha1, :sha256, :sha512, # hash type - common to all fetchers
-          :cookie, :warning, :unsafe, :extract, # used by net_fetcher
+          :cookie, :warning, :unsafe, :extract, :authorization, # used by net_fetcher
           :options, # used by path_fetcher
           :submodules # used by git_fetcher
         ]