Tweaks to the package metadata

Summary:
* Bump version to 0.0.5
* Have separate homepage and download URLs

Reviewed By: dkgi

Differential Revision: D7969392

fbshipit-source-id: c86c599af601d547d1eff8d0aaf103c7b73abcef

Number of workers must be positive

Summary: A negative number of workers is rejected, a zero-number of workers is automatically overridden.

Reviewed By: dkgi

Differential Revision: D7965777

fbshipit-source-id: d8741f103b9e57a8d928842a74bd35c19f234116

Move tracking of number of workers into configuration

Summary:
The previous code was storing the number of workers into the arguments
data structure, and adding this information at runtime. This would
cause scary backtraces if the code raised an exception before the
default configuration was applied.

This diff moves the attribute into the configuration class, where it
is easier to track its default and its lifecycle. This way, backtraces
are not printed anymore.

Reviewed By: shannonzhu

Differential Revision: D7921335

fbshipit-source-id: ec1578ae0e42dac334def34f641ede4caea8077e

Initial commit

fbshipit-source-id: e5d4e22fd99fa6b01c020be19b8c3a7843f08d86