Implementation of custom upload method in YUI HTML5 Uploader

Hi there,

this PR allows for a custom upload method for the YUI HTML5 Uploader. The upload method can be specified on instantiation of Y.Uploader via an additional (optional) Uploader configuration attribute, uploadMethod.
The default value ("POST") means the behaviour stayed the same as the current behaviour, but this new attribute allows for the Uploader to be used with PUT as well (eg. uploadMethod: "PUT").

Do let me know if you have any remarks - thanks in advance for merging this PR!

Kind regards,
David