Update Mesos C++ Style Guide for namespace usage

Discussed with bmahler , the current C++ style guide do not including the usage for namespace, we need to update the document to tell developers how to use namespace for future coding.

In general we avoid 'using namespace foo' statements as it is not explicit about which symbols are pulled in, and it can often pull in a lot of symbols, which sometimes lead to conflicts.

we're going to need namespace aliases to help pull in subnamespaces, e.g. namespace http = process::http; but this isn't in the style guide yet, we need to update the style guide to reflect this.