Dev/ck 1 add custom headers

This is for adding additional build tasks that can bulk-set Headers on uploaded files and directories:

  <UsingTask AssemblyFile="$(EmkayS3ClassLibrary)" TaskName="PublishFolderWithHeaders" />
  <UsingTask AssemblyFile="$(EmkayS3ClassLibrary)" TaskName="PublishFilesWithHeaders" />

  <ItemGroup>
    <ExampleItem Include="stylesheet">
      <Content-Type>text/css</Content-Type>
      <Content-Encoding>gzip</Content-Encoding>
    </ExampleItem>
  </ItemGroup> 

  <ItemGroup>
    <ExampleItem Include="stylesheet">
      <Content-Type>text/css</Content-Type>
      <Content-Encoding>gzip</Content-Encoding>
    </ExampleItem>
  </ItemGroup> 
  
<PublishFolderWithHeaders
	Key="$(YourKey)"
	Secret="$(YourSecret)"
	SourceFolders="@ExampleItem"
	Bucket="$(YourBucket)"
	DestinationFolder="$(YourDestination)" />

The test for the DoesS3BucketExist bug is not essential but helps to highlight an issue that may be hard to detect.