Fixed issue#93: Function.get_parameters returns empty list if there is only one parameter.

Tests as follows
```python
import sqlparse
function = sqlparse.parse('select abs(some_col) from some_table;')[0].tokens[2]
print function
print function.get_parameters()
```
Results:
```
abs(some_col)
[<Identifier 'some_c...' at 0x10ba97518>]
```