Initial pass at OFX imports

Still needs some fixups:
 * UI is incomplete
 * Investment transactions are unbalanced initially
 * OFX imports don't detect if one of the description fields for a
   transaction is empty (to fall back on another)
 * I'm sure plenty of other issues I haven't discovered yet

libofx.c

@@ -0,0 +1,14 @@
+#include <libofx/libofx.h>
+#include "_cgo_export.h"
+
+int ofx_statement_callback(const struct OfxStatementData statement_data, void *data) {
+	return OFXStatementCallback(statement_data, data);
+}
+
+int ofx_account_callback(const struct OfxAccountData account_data, void *data) {
+	return OFXAccountCallback(account_data, data);
+}
+
+int ofx_transaction_callback(const struct OfxTransactionData transaction_data, void *data) {
+	return OFXTransactionCallback(transaction_data, data);
+}