Add gin-gonic example

[hazcod]

Edit: not sure why the build fails, since I only modified the README.

[justinas]

Travis errors out on Go 1.0 and 1.1: these versions are probably not provided anymore by whatever Travis uses (seems to be this gimme thing).

[justinas]

The example should probably go under the "Example" section, not under "Manual token verification".

[ptman]

Maybe put in separate file in examples/ instead of having potentially all frameworks as examples in the README