Add default request accept header, configurable via config.requestMedia

Because [GitHub API server changes behavior](http://developer.github.com/changes/2014-01-07-upcoming-change-to-default-media-type/) and returns [slightly different content](http://developer.github.com/v3/versions/#differences-from-beta-version), it would be probably _best_ to have "beta mediaType" as default.

Can be configured per one call (`headers`), or with githubApi instance through `config.requestMedia`.

> Starting April 15, 2014, the beta version will no longer be the default version. However, we expect to continue supporting the beta version for a while.