Adds the upload asset release api

Here's a first pass at adding this feature. Please tell me if you'd
like it implemented a different way. Of course I like it as is :P
but some things to consider

*   Content-Type

    Currently I set the content type automatically based on the name
    you pass in. This is a little magic as it assumes there's both
    a `filePath` and `name` field.

    It could be required to be set by the user as in

        github.releases.uploadAsset({
            owner: owner,
            id: releaseId,
            headers: {
               "content-type": "application/zip",
            }
            ...

    Similarly the size could be manually set there. But, since, based
    on the current API there's no way for us to upload the file (we
    don't get the request object back, and we can't just pass the
    data (it's arguably too big. Could be 100s of meg, more than
    fits in node's memory space), it seems like we should lookup
    the size and stream the file as it does now?

*   Timeout

    Wasn't sure what to do about that. Timeouts don't seem appropriate
    for uploading giant files or at least can't easily be a global
    setting.

Anyway, if you don't like this patch hopefully you can suggest a way
you'd prefer it.
Add in support for a passed in upload url
Merge pull request #1 from kaanon/add-upload-asset

Add upload asset