introduce HTTParty-configuration, fix #61

This commit introduces a new option attribute, namely `httparty`, that allows passing configuration down to HTTParty.
For instance to disable verification of self-signed certificates you can now do as follow:

    client1 = Gitlab.client(endpoint: 'https://api1.example.com', private_token: 'user-001', httparty: {verify: false})

The list of HTTParty configuration attributes can be found at https://github.com/jnunemaker/httparty