services: incorporate a scheduling system

[needmorecowbell]

Some services will rely on other services to run before getting meaningful information. For example, the emlparser service should be run before everything, since it allows for other services to have an easy way to analyze the code coming in with python.

In addition, some services won't be able to run if conditions aren't met. Ie an eml may contain a zip file, which should have certain services run on it. One may extract the zip and once extracted, and then run services based off each file type within the zip.

Maybe services have conditions? Or an ordered list of dependent services?

Should there be a vault-wide service check that happens periodically?

Maybe some of these questions will be answered once more services are added. This may be answered when issue #3 is completed