Add support for GO 1.4 and older

This supersedes #1010 as it contains the full fix for supporting Go 1.4+ SDKs as well as <1.4 SDKs.

There is one more thing I'd like to change about how we handle SDKs in the app, more precisely, don't show the option to configure the GOROOT for IntelliJ IDEA IDEs but do show it for other IDEs. However I'll do that in a separate PR as my time is limited right now and it's not urgent nor it affects functionality.
@mtoader I'd basically like to remove the whole block that I'm commenting here right now as it's not used to the best of my knowledge (it deals with enumerating the .a files and we are not using those (neither here nor in the foreseeable future).

Any input on it?