Grequests rework & VirusTotal Speed Up & Add ShadowServer API Lookup

* Consolidate grequests code
The grequests code in output_filters is really pretty useful. However, there were multiple implementations of directly calling grequests with separate error handling and such. This change consolidates everything into `osxcollector.output_filters.util.http.MultiRequest`

* Speed up VT lookups
Around 25x faster by taking advantage of some of the smart things in `osxcollector.output_filters.util.http.MultiRequest`

* A bunch of moving around of code
Create the `osxcollector.output_filters.util` module. Move a lot of helper functions used by multiple filters into this module.

* Add ShadowServer filter
ShadowServer helps identify known good files. Use it.