Add _some_ Behavioural Cohort methods to AmplitudeAPI

[asibs]

https://www.docs.developers.amplitude.com/analytics/apis/behavioral-cohorts-api/

Adds 3x new methods to AmplitudeAPI:

• get_cohorts - gets all Behavioural Cohorts, returns the data in the Faraday response object - a basic example of using this API and parsing the response has been added to the README
• add_to_cohort - adds a list of users to a specific Behavioural Cohort
• remove_from_cohort - removes a list of users from a specific Behavioural Cohort

Other changes

• Factored out the call to get a connection with basic auth to a private helper, so it can be re-used in the new Behavioural Cohort methods
• Added a bin/console script to make it slightly easier to manually run the gem code in an IRB session

[AlvaroMaceda]

Great, @asibs, thank you very much for your contribution.
Could you add some specs for the new API? It will be great to have all the functionality covered.