M1456, Implement MIME sniffing initial Step
auto merge of #3766 : t29/servo/mime-sniffing, r=jdm

Issue: #3144 

We created a sniffer task in components/net/, added a call in resource_task load function to create a new sniffer task (sending all the data), and sniffer_task currently sends all the data back to resource_task.

The purpose of this request is to get feedback from @jdm on our progress before moving forward and writing tests.