Added ability to use beta api, intergrated 5 options

[apankDocero]

Beta API provides better website rendering.
With the current API I sometimes run into svg issues even though it renders correctly on even IE 9.

To use simply add the 'beta' string at the end
var url2png = require('url2png')('API_KEY', 'PRIVATE_KEY', 'beta');

Integrated 5 options 'user_agent', 'unique', 'accept_languages', 'ttl' and 'custom_css_url'.

Current know issues with both the current + beta API's:
When either the 'user_agent' or 'accept_languages' is specified it will only accept url's with out 'http/https' and without a subdirectory. Else it will throw a 500 error and return this -

Reaching out to support to see if they can help resolve that issue.

[ForbesLindesay]

I'm no longer using/maintaining this library. Let me know what your npm user name is and I'll add you as an owner on npm so you can publish new versions.

P.S.

I'd recommend an api that accepts an options object like {version: 'beta'} instead of just the 'beta' string literal as this will allow for more options in the future if need be.

[apankDocero]

Thanks! My npm username is apank

[ForbesLindesay]

Sorry for the delay, you are now an owner of the npm package and ca collaborator on this GitHub repo. Feel free to merge and publish this once you've made it take an options object rather than a plain string.

[leslie-alldridge]

If this still needs doing @ForbesLindesay I wouldn't mind helping out - cheers