Add API.md documenting all available methods

[anandthakker]

Quoting @artemp from https://github.com/mapbox/variant/pull/156#discussion_r127658875:

it'll be better to have a separate page listing all available methods while README only mentions basic building blocks

[lucaswoj]

Just had a quick phone call with @mapsam and @anandthakker to discuss this. We're going to band together to write a lightweight API.md that

• lists all methods
• provides a 1-2 sentence summary of each (can draw upon http://en.cppreference.com/w/cpp/utility/variant in many cases)
• provides an inline example of each
• links to available open source documentation for each