Major rework of codebase

- Get rid of 'core' and move CLI to cmd/gas directory
- Migrate (most) tests to use Ginkgo and testutils framework
- GAS now expects package to reside in $GOPATH
- GAS now can resolve dependencies for better type checking (if package
  on GOPATH)
- Simplified public API

testutils/log.go

@@ -0,0 +1,12 @@
+package testutils
+
+import (
+	"bytes"
+	"log"
+)
+
+// NewLogger returns a logger and the buffer that it will be written to
+func NewLogger() (*log.Logger, *bytes.Buffer) {
+	var buf bytes.Buffer
+	return log.New(&buf, "", log.Lshortfile), &buf
+}