Improvements

[roelderickx]

See issue #3:

• A function log_browser_console is added to add log messages in the browser console. Always helpful while developing.
• encode_message is integrated in send_message. I know where you got the code from but it requires the user of the library to know what kind of input send_message exactly requires. This is not clearly documented, except that encode_message must be called. It simply looks superfluous in the code using the library.
• An install() function is added. I would like the user of my software to be able to simply click 'Enable for Firefox' or 'Enable for Chrome' in stead of executing nativemessaging-install.py manually. Of course a manual installation is still possible with my version, it just calls the same install procedure.