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.
For this particular usage yes. We've been successfully using this in master since a few years and we never had complaints from users around the world. However, I'm open to suggestions if it can be improved.