Add composer.json to make installable via composer

Signed-off-by: Daniel Martins <daniel.martins@lumedigital.com.br>

Reference to issue [#112](thomasgriffin/TGM-Plugin-Activation#112)

To test it locally, add this to one of your projects with composer:
```json
{
"require": {
        "tgm/plugin-activation": "@dev"
    },
    "repositories": [
        {
            "type": "vcs",
            "url": "/Users/danielmartins/src/TGM-Plugin-Activation",
            "source-reference": "7d6093ebfeb1eeae42161c429a5d724790509970"
        }
    ]
}
```