Support image mime types

This adds another specific route for image/* mime types
Maybe could be unified a bit with the multipart handling
in that code section. Creating a parser in the user code
depends heavily on the .emit('end') sequence if it works
or not...
Add content type image/png test case
Fixed: error parsing body of 204 response
default user agent
Added .remove() header method, tests #342
remove() -> unset()
Remove 0.9 from tests
Dump dependency version for cookiejar.

The `cookiejar` dependency is out by 2 minor versions, which solve a few bugs in that package such as https://github.com/bmeck/node-cookiejar/pull/6

The author has recently `published` a new version to `npm`: https://github.com/bmeck/node-cookiejar/issues/12

fixes: https://github.com/visionmedia/superagent/issues/174
client: add basic `field()` and `attach()` functions

Just really basic wrappers around the HTML5 `FormData` object.
Merge pull request #373 from TooTallNate/add/client-form-data

client: add basic `field()` and `attach()` functions
node: ability to .attach() a `fs.ReadStream` instance

This allows us to do, i.e.:

``` js
request.post('/upload')
  .attach('file', fs.createReadStream('foo.txt'))
  .end(fn);
```

Eventually it would be better to add support for arbitrary
`stream.Readable` instances, but this is good enough for now.
Merge pull request #371 from TooTallNate/update/add-fs-readstream-support

node: ability to .attach() a `fs.ReadStream` instance
Merge pull request #369 from missinglink/patch-1

Bump dependency version for cookiejar.
mv pkg declaration
Merge pull request #342 from bevacqua/master

Set a default user agent
node: deprecate the Part() constructor

The new `.attach()` interface should make this unnecessary.
node: deprecate `Request#part()`

Same as the `Part()` constructor, should no longer be
necessary with the new `attach()` interface.
node: proper inheritance

This was just annoying me...
node: use "form-data" module for multipart requests

Simplifies the logic in superagent greatly, and delegates the task
off to another module like it should be. The "form-data" module
has a lot better Stream support, and should overall be a lot more
reliable than the hand-rolled logic.
node: add backwards-compat for the `Part` class

Ideally nobody will even be using this... but this way, if anyone
is depending on the `.part()` function should still have it work
as expected.