[PATCH] Add missing type conversion functions for PL/Python

PL/Python already has different type conversion functions to

convert PostgreSQL datum to Python object. However, the conversion

functions from Python object to PostgreSQL datum only has Boolean,

Bytea and String functions. 

In this patch, we add rest of Integer and Float related datatype conversion functions

and can increase the performance of data conversion greatly especially

when returning a large array.

We did a quick test about the performance of returning array in PL/Python:

The following UDF is used for test:

```

```

The test command is 

```

```

The count is used for avoid large output, where n is the number of element in returned array, and the size is from 1.5k to15m. 

Size of Array      1.5k           |            15k         |           150k        |           1.5m        |         15m       |

Origin                 2.324ms     |     19.834ms      |     194.991ms    |     1927.28ms    |   19982.1ms  |  

With this patch   1.168ms      |      3.052ms       |      21.888ms     |     213.39ms      |    2138.5ms   |

All test for both PG and PL/Python are passed.

Thanks very much.