Fix to return correct microsec for timestamp datatype

[shabbir10july]

JIRA: #37

Problem:
nzpy returns Incorrect timestamp for below data in table :

DB1.ADMIN(ADMIN)=> select * from date_Table;
 ID |         DATE_DATA
----+----------------------------
  2 | 2012-10-15 12:57:18.000396

nzpy returns :

[nz@sim-ips26 python]$ python3 run4.py
['2012-10-15 12:57:18.396']

Solution:

1. Added zero padding before microsec
2. Removed unwanted code "fraction *= 1000"
3. In few cases, dt param in timestamp2struct() is negative which calculates wrong date. Corrected division there.

Testing: Ran above test case. Ran nzpy testcases and nzalchemy+python testcases.