Add F-Droid as a PURL type

[armijnhemel]

it might be useful to add a PURL type for F-Droid. Currently they have ~8000 packages or so, with corresponding source code.

[pombredanne]

This is at https://www.f-droid.org/

[armijnhemel]

The XML release format is described at:

https://f-droid.org/en/docs/Build_Metadata_Reference/

and in the XML file itself:

https://f-droid.org/repo/index.xml

(warning: might take some time to load)

[pombredanne]

This PR is adding fdroid support in the purldb at https://github.com/nexB/purldb/pull/10 and becomes a good way to specify the fdroid purl type