CI to UPM branch

[Hermesiss]

Added project for convenient edition
Added travis-ci.org integration
Added setup instructions to README.md

This CI will push changes to branch upm (will be created in process), so users could use it in UPM without downloading files as described here. You can skip ci if it's not necessary by adding [skip ci] in comment message.

It also will make branches upm-dev and upm-test for develop branch and pull-requests

I'm using this in my work, e.g. https://github.com/Hermesiss/unity-splines

I've tested it in my fork, in editor and build mode (no errors)

You can test it as package adding this line to your Packages/manifest.json

"asset-bundle-manager": "https://github.com/Hermesiss/AssetBundleManager.git#upm-dev"

Before merging you need to:

1. Sign up at Travis Ci
2. Enable this project there
3. Generate github access token with all repo scopes
4. Add this token to Environment Variables in settings