Initial commit of the Spring Boot plugin.

Hi,

First of all please try not to laugh because of my JavaScript skills :P This is my very first JavaScript code, so sorry for low-quality code. My JS skills will improve over time with next pull requests :) .

This is the very first version of the Spring Boot plugin. For now it only: 
- detects if container to which Hawt.io is connected is Spring Boot app
- if the above is true, it adds extra 'Spring Boot' tab to menu
- the only thing I display now is the user-friendly version of `/metrics` stats

I can already think about a million of enhancements for Spring Boot plugin. I'll add them in the subsequent pull requests.

Any feedback regarding the plugin itself and my JS skills will be highly appreciated.

Cheers :) .

/CC @jstrachan