This commit adds a mechanism for changing the backend used for running the chisel3 test suite

It will check for the presense of file in the chisel3 root directory named TestBackendName
If not found if test will default to the Verilator backend
If it is found it will look for lines with either the word Treadle or Verilator and
use the corresponding backend.
Convenience tasks
setTreadleBackend    sets backend to treadle
setVerilatorBackend  sets backend to verilator
setDefaultBackend    removes TestBackendName file which effectively sets backend to verilator

The file name TestBackendName has been added to .gitignore
The verilator backend is the historical backend and currently defines expected behavior
The treadle backend is experimental and runs much faster