Proof-of-concept bid request functionality

Doesn't allow user to set max price, or even allow traditional instance
creation, but demonstrates the principle.
First shot at UI for bidding.

Still need to put current price, etc, on there
Polish UI for spot instances, and show current price
Bid doesn't need cancelling under normal circumstances
Merge branch 'feature/spot' into develop
Insert (untested) CancellationTokens into Ec2Manager.CreateAsync
Add tokens to volume creation codepath
Working cancellation for instances and volumes
Merge branch 'feature/cancellation' into develop

* feature/cancellation:
  Working cancellation for instances and volumes
  Add tokens to volume creation codepath
  Insert (untested) CancellationTokens into Ec2Manager.CreateAsync
Warn user if spot bid price is too low
README corrections