Add support for a DropboxFile from stream

[jnstr]

Sometimes we need to upload a file that isn't on the filesystem but is already a file stream.
This commit adds a new static function CreateByStream that allows us to create a new DropboxFile instance using a stream.

You can now instantiate a new DropboxFile using these static functions:

// create new DropboxFile from stream
$dropboxFile = DropboxFile::createByStream($fileName, $fileStream, DropboxFile::MODE_READ);

// create new DropboxFile from path (same as the constructor but just added in order to match the createByPath function)
$dropboxFile = DropboxFile::createByPath($filePath ,DropboxFile::MODE_READ);

[kunalvarma05]

@jnstr Thanks for the contribution :)