External file save

I would like to send in this pull request with 2 commits that:

* checks for the ratio of the loaded image (being it by load or by intent) against the current screen ratio. If necessary, it rotates the bitmap in order to best fit the drawing canvas. 
* adds the possibility to pass through the intent's extra bundle by means of the key *MediaStore.EXTRA_OUTPUT* (not sure if that is the right choice though) a path to which the image should be saved to when pushing the save button. If that path if supplied, the saving to temporary files is overridden. For that I had to extract the retrieval of the file path into a separate method, to be able to check the case of output path availability.

It would be great to have the possibility to use markers from within a third party application.

Let me know if the request is of interest and if there is something that needs to be changed.