add simple example app to show how to use it

[mdamien]

it doesn't use yet the advanced features like:

• adding new emails
• token expiration

But it's a first start to fix #30, #29 and #23 (via this request for better docs)

[coveralls]

Coverage remained the same at 94.444% when pulling d021cd2 on mdamien:add-example-app into 74a1b09 on mfogel:develop.