Fix: Flatten extracted archives analogous to cocoapods-downloader

[rhodes73]

When a universal package contains a single tar archive, this archive will automatically be extracted. This is great, however for tar archives (among others) it's common that they contain a single directory with the same name as the tar file (without the .tar or .tar.gz extension). The extract_with_type function defined in the cocoapods-downloader module usually takes care of extracting the children of such folders, in order to create the correct structure for installing the CocoaPod. However, for this to work, we have to set some properties/options before calling extract_with_type:

• filename: This property is used by the extract_with_type function to distinguish the archive from the extracted directory (see https://github.com/CocoaPods/cocoapods-downloader/blob/master/lib/cocoapods-downloader/remote_file.rb#L109).
• type: Whether an extracted directory should be flattened is decided according to the type of its corresponding archive. If we don't set the type explicitly, cocoapods-downloader will determine the type automatically from the url property, which in case of universal packages, does not contain the .tar or any other archive type extension. We therefore need to set the type explicitly for the flattening to work (see https://github.com/CocoaPods/cocoapods-downloader/blob/master/lib/cocoapods-downloader/remote_file.rb#L52 and https://github.com/CocoaPods/cocoapods-downloader/blob/master/lib/cocoapods-downloader/remote_file.rb#L30)

[pallavjosh]

@rhodes73 are you able to use it? #5 i have created this issue, readme not clear to me..where to change podspec file (in source repo or consuming repo)? please share if any working sample available