add decimal support

Hi,

please review the code and tell me what you think about it.

Usage example in the shell:

```erl-sh
159> bank:prepare(test_db, insert_t3, "INSERT INTO t3 (amount) VALUES (?)").
ok
160> bank:prepare(test_db, select_t3, "SELECT * FROM t3").                  
ok
161> bank:execute(test_db, insert_t3, [{decimal, 12000, 5}]).               
{ok,1,13}
162> bank:execute(test_db, select_t3, []).                                  
{rows,[[{<<"id">>,1},{<<"amount">>,{decimal,3333350000,5}}],
       [{<<"id">>,2},{<<"amount">>,{decimal,9999999999,5}}],
       [{<<"id">>,3},{<<"amount">>,{decimal,1000000000,5}}],
       [{<<"id">>,4},{<<"amount">>,null}],
       [{<<"id">>,5},{<<"amount">>,{decimal,-100000,5}}],
       [{<<"id">>,6},{<<"amount">>,{decimal,-1,5}}],
       [{<<"id">>,7},{<<"amount">>,{decimal,0,5}}],
       [{<<"id">>,8},{<<"amount">>,{decimal,-1,5}}],
       [{<<"id">>,9},{<<"amount">>,{decimal,-1000000,5}}],
       [{<<"id">>,10},{<<"amount">>,{decimal,-100000,5}}],
       [{<<"id">>,11},{<<"amount">>,{decimal,-10000,5}}],
       [{<<"id">>,12},{<<"amount">>,{decimal,-10000,5}}],
       [{<<"id">>,13},{<<"amount">>,{decimal,12000,5}}]]}
```

This is when you query MySQL directly:

```
mysql> select * from t3;
+----+-------------+
| id | amount      |
+----+-------------+
|  1 | 33333.50000 |
|  2 | 99999.99999 |
|  3 | 10000.00000 |
|  4 |        NULL |
|  5 |    -1.00000 |
|  6 |    -0.00001 |
|  7 |     0.00000 |
|  8 |    -0.00001 |
|  9 |   -10.00000 |
| 10 |    -1.00000 |
| 11 |    -0.10000 |
| 12 |    -0.10000 |
| 13 |     0.12000 |
+----+-------------+
13 rows in set (0.00 sec)
```
'matthias'.