Add support for attributes that aren't strings

By default, all attributes when syncing a lead record default to being caused a string. This causes problems for non-string attributes - for example, if you try to set a boolean attribute, it won't work.

This update adds support for this by adding an extra attribute to `Markety::LeadRecord.set_attribute`:

```ruby
lead.set_attribute("Company", "Acme Industries") # Type defaults to string...
lead.set_attribute("Activated", true, "Boolean") # ...or you can set a custom type of your choice!
```

I've also added a Gemfile, which means you can install all the dependencies just by running `bundle`.