Provide public method to set SampleRate for Raven Client.

[SampleRate](https://godoc.org/github.com/getsentry/raven-go#SetSampleRate)

Currently the methods allowed are 

- `SetRelease`
- `SetEnvironment`
- `SetServerName`

A public method for setting the `SampleRate` would be useful.

I can take this up.
Thanks.