Executor and JobDescription JSON support added

I added the implicit json writers for Executor and JobDescription and modified the jobId query. 

The updated jobId query is optimal because:
1. Array.find method is called on MasterState's completedJobs only if no matching job is found in activeJobs
2. Creating a new collection from the ++ operation is avoided

The downside is that the updated version is arguably less concise and more lines of code. Overall I believe a positive.