Add default operation id on client creation

[jondmcelroy]

When I started using this library I didn't know that you needed to set the operation id in order to link calls. I added it by default since I'm not sure anyone would want this without it. I also updated the documentation to be more clear about the other possible options.

• updated test
• README updated with more examples of setting different context configurations and changing operation id
• operation id is automatically generated as a guid on context creation

[jjindrich]

I checked your PR, but I don't see how you setup Operation_name. I documented on my GitHub https://github.com/jjindrich/jjazure-web-php/blob/master/jjwebphpai/src/index.php. But I'm not sure what is exactly required to have working all reports.

[jondmcelroy]

@jjindrich Yes, this PR is only looking at the Operation ID, not the name.