feat(util/http_client): Adds generic HTTP client

Supports retries with exponential backoff as well as connection
timeouts and the ability to skip SSL/TLS verification.

This commit also refactors datasource and initialize packages
in order to use the new HTTP client.
style(util->httpbackoff): Changes package as per @philips suggestion
style(httpbackoff -> pkg): Adjusts package name to follow convention
fix(pkg/http_client): Printf is smarter than you think

Printf determines what the duration unit is
and prints it accordingly.
Merge pull request #123 from c4milo/shared-http-client

feat(util/http_client): Adds generic HTTP client