Add go1.4 support

This adds support for Go 1.4+ SDKs.

I've also corrected the default Linux paths for the SDK (as per [Go documentation](http://golang.org/doc/install#tarball)) and upgraded the suggestions for the SDK default directory.

After the change I've noticed a severe drop in performance due to ``` getPathsToLookup() ``` method being called a great deal of times / file / element ``` src/com/goide/psi/impl/imports/GoImportReferenceHelper.java ```. Example to be used: ``` src/fmt/print.go ``` from Go 1.4 SDK.
Extracting ugly map instead of simple comparison doesn't look like 'long run' to me.
If Go team will introduce yet another difference in path for later version, we would discuss about extracting this logic in some enum or Version-entity. For now it's overkill.