add slidescore url and slidescore study id to download manifest.

[YoniSchirris]

• save manifest as json
• when starting the download, add slidescore_url and slidescore_study_id
• add layer of slidescore_url and slidescore_study_id keys, might a user download multiple studies into a single directory, so as to not overwrite the initial values of slidescore_study_id if a second study is downloaded to the same directory.

Fixes #40

[YoniSchirris]

Now also fixes #43