Add CDB_SetUserQuotaInBytes function

Closes #2

test/CDB_QuotaTest.sql

@@ -0,0 +1,11 @@
+set client_min_messages to ERROR;
+\set VERBOSITY terse
+CREATE TABLE big(a int);
+SELECT CDB_CartodbfyTable('big');
+INSERT INTO big SELECT generate_series(1,1024);
+SELECT CDB_SetUserQuotaInBytes(8);
+INSERT INTO big VALUES (1);
+SELECT CDB_SetUserQuotaInBytes(0);
+INSERT INTO big VALUES (1);
+DROP TABLE big;
+set client_min_messages to NOTICE;

test/CDB_QuotaTest_expect

@@ -0,0 +1,10 @@
+SET
+CREATE TABLE
+
+INSERT 0 1024
+0
+ERROR:  Quota exceeded by 103.9921875KB
+8
+INSERT 0 1
+DROP TABLE
+SET