adding support for other HTTP methods in sendTo

**tl; dr: I added an `options` object to `sendTo` and `sendDataReadyTo` which allows for specifying a method other than `POST`**

Thanks for the library!

## my problem
I am using [jquery.dav](sandro-pasquali/jquery.dav) to work with files on a WebDAV server (i.e. SharePoint). I have limited visibility into the server permissions, and my files can be large, so my approach is to "fail fast":

- try to create a file (with `PUT`) 
  - react if it fails
- `PUT` the file contents (replacing the empty contents)

At first, I tried to use `file.readData`, and put the content up myself, but was getting poor results with binary files... it is likely I was using `readData` incorrectly, or don't understand how to use the File API. Further, for large files, I'd like to avoid reading the data into memory unless absolutely necessary.

## my hack
However, hacking the filedrop source directly to use `PUT` solved my problems, for both text files and binary files, so I went ahead and monkeypatched `sendDataReadyTo` in my code to do what I want, but would like to see this option as part of the filedrop itself, if possible.

However, having only started using filedrop, I am not sure of the right way to add this to the API, since `POST` is specified as part of the `xhr.open` method, and cannot be overridden with `File.event('xhrSetup')` or `File.hookXHR`. The easiest is probably to just add `method` argument to `.sendTo` (and therefore `.sendDataReadyTo`).

## my solution
This PR takes a somewhat-more liberal position, and adds an `options` object to the key methods... it looks like only `async`, `username` and `password` would be used otherwise in this exact way, and those are unlikely to be very useful, but perhaps there are other possibilities I haven't thought of.

## testing
I notice that `sendTo` does not appear in the test suite. I could support setting up a `.travis.yml` to add some testing against a WebDAV server (I use [WsgiDav](https://github.com/mar10/wsgidav) for automated testing), if that would help get this feature added.

## related
a successful `PUT` is returning a `204 No Content`, for which I am having to use
````.js
file.event("error", function(xhr){
  return [xhr.status === 204 ? success : fail](xhr);
})
````
Which seems a bit odd, but I am not sure how much HTTP-fu needs to be baked in... perhaps another day...

Thanks again!