Expose an API for Shaarli

[ArthurHoaro]

Following the discussion in #586 and in https://github.com/shaarli/api-documentation, Shaarli will expose an API to allow third party to interact with Shaarli's data when authenticated (e.g. mobile app).

The milestone for this is 0.8.x.

Tasks:

• Define a readable API definition. See https://shaarli.github.io/api-documentation [Status: almost done]
• Make the API definition easily accessible from Shaarli's documentation. [Status: TODO]
• Document the API authentication process [Status: done]
• Add the API authentication documentation in Shaarli's documentation. [Status: TODO]
• Provide a basic API implementation able to process request and render response. [Status: almost done]
• Add the API secret in the administration [Status: almost done]
• Implement GET /links service. [Status: TODO]
• Implement POST /links service. [Status: TODO]
• Implement GET /links/{id} service. [Status: TODO]
• Implement PUT /links/{id} service. [Status: TODO]
• Implement GET /history service. [Status: TODO]
• Implement GET /infos service. [Status: almost done]
• URL Rewriting for nginx documentation
• URL Rewriting for lighthttp documentation