Multiple features and enhancements

[kishaningithub]

Thank you for taking the time to port node js version of netlify-cms-oauth-provider to go lang :-)

When trying to integrate this with my blog i faced several huddles. Have fixed several of them and added several cool features. Do check and let me know

Summary of features/fixes

• [x] Usage of go modules
• [x] Fix for #4 using new CALLBACK_HOST env variable
• [x] Travis CI integration
• [x] Go releaser and go downloader integration. Fixes #3
• [x] Use of log package instead of fmt to print info messages
• [x] Replace custom dotenv with the awesome godotenv library.
• [x] Fix bitbucket callback

Also have taken the liberty of adding MIT license as the repo did not have any license. Hope its fine.