Refactored sentryRequest creating into own method. Exposed CaptureMessage()

Hi,

I found it rather frustrating to not be able to define the loglevel or logger object, so refactored the code to generate the `sentryRequest` a bit. I also added a `CaptureMessageA()` function where one can provide the level as well as the logger.

I am not sure about the name, but couldn't come up with a better one. Any ideas? :)

Cheers
 ZeissS