Refactor out build notifications into separate plugin

Refactor build notifications into separate plugin and core mail sending functionality into separate NPM module.

This pull request has a few dependencies.

strider-simple-runner needs to have strider-runner-core updated to a version which includes https://github.com/Strider-CD/strider-runner-core/pull/1

strider needs to have strider-simple-runner updated to a version which includes the strider-runner-core update and: https://github.com/Strider-CD/strider-simple-runner/pull/23

I have split out the core mail sending functionality of strider into its own module and added a direct link to my Github repo for the time being. I imagine you will want to retain control of the ownership of this code and its corresponding NPM module (as it is such core functionality), so I haven't published this (unless you are happy for me to do so and add yourselves as owners? In which case I will update the package.json to include the actual NPM module). Feel free to fork the repo to your own and I will remove mine, or however else you would like to take ownership of it.

I have published strider-email-notifier to NPM, available here: https://npmjs.org/package/strider-email-notifier

I wasn't sure if you would want this as a default module within strider, so I have not included it as one in this PR. If you would like it as a default, let me know and I will update the package.json to include it.

I think thats it. Let me know if you have any questions or comments on the changes!