Add support for compound queries

This commit adds support for the compound queries "bool", "boosting",
"constant_score" and "dis_max". The "function_score" query is not
supported yet.

Compound queries are simple. They act just like simple queries, except
that they are recursive, wrapping other simple/compound queries.

For example:

    esquery.Bool().
        Must(Term("user", "kimchy"), Term("author", "kimchy")).
        Filter(Term("tag", "tech"))

boosting_test.go

@@ -0,0 +1,18 @@
+package esquery
+
+import (
+	"testing"
+)
+
+func TestBoost(t *testing.T) {
+	runTests(t, []queryTest{
+		{
+			"boosting query",
+			Boosting().
+				Positive(Term("text", "apple")).
+				Negative(Term("text", "pie tart")).
+				NegativeBoost(0.5),
+			"{\"boosting\":{\"positive\":{\"term\":{\"text\":{\"value\":\"apple\"}}},\"negative\":{\"term\":{\"text\":{\"value\":\"pie tart\"}}},\"negative_boost\":0.5}}\n",
+		},
+	})
+}