Add go1.4 support

This adds support for Go 1.4+ SDKs.
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.