Let dashing respond to signals (i.e. SIGTERM)

Above all this makes docker stop work correctly.
Supply custom widget IDs upon docker run.

Use syntax:
  docker run -P frvi/dashing <GIST_ID> <GIST_ID> <GIST_ID> ...

This change sets the entrypoint in the dockerfile which makes
it impossible to get a shell in the image by just supplying
/bin/bash as an argument to run. Use the --entrypoint option
instead (see section "Debugging" in README.md).
Expose dashing configuration as a seperate volume.
Cleaning up README.md
Merge pull request #1 from mattgruter/master
Cleaned up comment in Dockerfile.
Now with custom gemfile dependencies.

Custom widgets supplied via env variable instead of command line arguments.
Interactive mode "back to normal".
Only install widgets and gems once.