Made the 'unf' dependency optional.

If you don't have 'unf' installed and you use an AWS adapter, you'll get a message about how your strings might not be properly escaped.  If the gem is available, we'll use it.

fog.gemspec

@@ -50,7 +50,6 @@ Gem::Specification.new do |s|
   s.add_dependency('net-ssh', '>=2.1.3')
   s.add_dependency('nokogiri', '~>1.5')
   s.add_dependency('ruby-hmac')
-  s.add_dependency('unf')
 
   ## List your development dependencies here. Development dependencies are
   ## those that are only needed during development

lib/fog/aws.rb

@@ -1,7 +1,12 @@
 require 'fog/core'
 require 'fog/aws/credential_fetcher'
 require 'fog/aws/signaturev4'
+
+begin
   require 'unf/normalizer'
+rescue LoadError
+  Fog::Logger.warning("Unable to load the 'unf' gem. Your AWS strings may not be properly encoded.")
+end
 
 module Fog
   module AWS
@@ -87,7 +92,7 @@ module Fog
     end
 
     def self.escape(string)
-      string = ::UNF::Normalizer.normalize(string, :nfc)
+      string = defined?(::UNF::Normalizer) ? ::UNF::Normalizer.normalize(string, :nfc) : string
       string.gsub(/([^a-zA-Z0-9_.\-~]+)/) {
         "%" + $1.unpack("H2" * $1.bytesize).join("%").upcase
       }

lib/fog/aws/storage.rb

@@ -170,7 +170,7 @@ module Fog
 
         # NOTE: differs from Fog::AWS.escape by NOT escaping `/`
         def escape(string)
-          string = ::UNF::Normalizer.normalize(string, :nfc)
+          string = defined?(::UNF::Normalizer) ? ::UNF::Normalizer.normalize(string, :nfc) : string
           string.gsub(/([^a-zA-Z0-9_.\-~\/]+)/) {
             "%" + $1.unpack("H2" * $1.bytesize).join("%").upcase
           }