adding log capability to client

[pawelsz-rb]

Description of the change

Making logging capability on the client .

Now sending messages works on both package (rollbar) and client levels.
example:

import "github.com/rollbar/rollbar-go"

rb_client := rollbar.New(...)
// stuff happens
rb_client.Log("error", err, request, ...)
// or 
rb_client.Error(err, request, ...)

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Maintenance
• New release

Related issues

• Fix Make Log a method of Client #110

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers assigned
• Issue from task tracker has a link to this pull request
• Changes have been reviewed by at least one other engineer

[matux]

This looks good to me, just a few very minor comments.

[matux]

Looks great, thank you @pawelsz-rb!