Add module execution status to output

[anthonysena]

When a module completed a successful execution in Strategus v0.x, an empty file named done was written to the module's results directory. This was a handy way for an end-user to know if a given module executed successfully. This behavior was removed in Strategus v1.x since we return the execution status as part of calling Strategus::execute. However, the execution status is lost unless you explicitly save it.

I'd propose saving the execution status information to a .json file that lives in the results folder. This would have the following benefits:

1. We can use this information to determine what modules require a re-execution due to a failure (probably need to check the analysis specs haven't changed for this behavior).
2. When this file is present, we can determine what results are ready for upload.
3. It may be useful to upload these results with other study results just for performance profiling.

[anthonysena]

Linking this to #79 since we may be able to bypass re-running a module based on the execution status described in this issue.