Support SOAP

[eyalkoren]

Client: do we look at specific SOAP clients, or just extend our general HTTP support (e.g. instrumenting HttpUrlConnection)

Server: naming SOAP-related transactions properly

[felixbarny]

related to #460

[eyalkoren]

Test the current support status, maybe major clients will be supported with HttpUrlConnection and other clients we already instrument. Maybe find a generic way to name spans properly

[eyalkoren]

Showed up in 30% of users in the survey

[alvarolobato]

This may be working with basic http headers support, needs testing.
We should also investigate if we want to do a follow up with soap:header for better compatibility with ESBs, etc.