Adds start_server helper for starting slowhttp server
Updates README with information on command-line flags and format for playback commands
Prevents bench from crashing when no output file is given
Has bench panic when rooturl parameter is not provided
Updates README to describe building bench executable
Updates bench test for no output
Fixes start_server error handling, indentation, and trailing newline
Moves start_server to slowhttp folder and makes necessary package updates
Adds .gitignore to ignore executables
Fixes start_server error handling
Merge pull request #1 from Clever/INFRA-111-convert-logs-to-gobench-input

Some README updates and minor improvements from working on INFRA-111