Disable console logging by default

[sjdemartini]

Description
Turns off logging to the console by default. Logging can still be re-enabled by calling adapter.disableLog(false).
Resolves https://github.com/webrtc/adapter/issues/68.

Purpose
Logging by default is not desirable functionality. For an example of not logging to the console being standard convention, see this linting rule from ESLint: http://eslint.org/docs/rules/no-console. Here is an excerpt:

In JavaScript that is designed to be executed in the browser, it’s considered a best practice to avoid using methods on console. Such messages are considered to be for debugging purposes and therefore not suitable to ship to the client. In general, calls using console should be stripped before being pushed to production.

As such, having logging enabled by default in adapter.js (which therefore logs the first "switch" statement of code in the browser, without the ability to turn it off) is fairly bad non-standard practice, and it requires editing the source or forking the project before using the code in production. The adapter code should be as widely usable as possible (and of course designed for the browser), so logging should be off by default.

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[sjdemartini]

I signed it!

[googlebot]

CLAs look good, thanks!

[fippo]

this was supposed not to log when used as a module but that seems to have broken recently.
PR is LGTM

[KaptenJansson]

LGTM

[sjdemartini]

Thanks for the quick review and merge @fippo and @KaptenJansson! Do you know when the next release is expected to be cut?

[KaptenJansson]

I will try to publish it today.