cmd/bosun: Button to download config to local machine

[captncraig]

Relies on browser's save dialog. Intent is user can save wherever they choose and upload to git or server or whatever.

[gbrayut]

Tested locally and works as expected but TravisCI is complaining:

Running go generate bosun.org/...
2015/07/14 17:01:17 running tsc
web/static/js/config.ts(400,3): error TS2304: Cannot find name 'saveAs'.
2015/07/14 17:01:19 run error: tsc: exit status 2
2015/07/14 17:01:19 run complete: tsc
2015/07/14 17:01:19 running esc
2015/07/14 17:01:20 run complete: esc
Go generate needs to be run. The following files have changed:
cmd/bosun/web/static.go

[kylebrandt]

Looks okay to me once it passed the build.