Adjust defaults for 'timestop' and search query.

[jmhardison]

I encountered errors with endpointtracker deploying tables to a new DB, which results from an error:

Traceback (most recent call last):
File "./aci-endpoint-tracker.py", line 276, in
main()
File "./aci-endpoint-tracker.py", line 272, in main
tracker(args)
File "./aci-endpoint-tracker.py", line 120, in tracker
c, cnx = connect_mysql(args)
File "./aci-endpoint-tracker.py", line 101, in connect_mysql
timestop TIMESTAMP);''')
File "/usr/local/lib/python2.7/dist-packages/PyMySQL-0.7.9-py2.7.egg/pymysql/cursors.py", line 166, in execute
result = self._query(query)
File "/usr/local/lib/python2.7/dist-packages/PyMySQL-0.7.9-py2.7.egg/pymysql/cursors.py", line 322, in _query
conn.query(q)
File "/usr/local/lib/python2.7/dist-packages/PyMySQL-0.7.9-py2.7.egg/pymysql/connections.py", line 835, in query
self._affected_rows = self._read_query_result(unbuffered=unbuffered)
File "/usr/local/lib/python2.7/dist-packages/PyMySQL-0.7.9-py2.7.egg/pymysql/connections.py", line 1019, in _read_query_result
result.read()
File "/usr/local/lib/python2.7/dist-packages/PyMySQL-0.7.9-py2.7.egg/pymysql/connections.py", line 1302, in read
first_packet = self.connection._read_packet()
File "/usr/local/lib/python2.7/dist-packages/PyMySQL-0.7.9-py2.7.egg/pymysql/connections.py", line 981, in _read_packet
packet.check_error()
File "/usr/local/lib/python2.7/dist-packages/PyMySQL-0.7.9-py2.7.egg/pymysql/connections.py", line 393, in check_error
err.raise_mysql_exception(self._data)
File "/usr/local/lib/python2.7/dist-packages/PyMySQL-0.7.9-py2.7.egg/pymysql/err.py", line 107, in raise_mysql_exception
raise errorclass(errno, errval)
pymysql.err.InternalError: (1067, u"Invalid default value for 'timestop'")

The value of '0000-00-00 00:00:00' is not according to MySQL docs a valid range of timestamps, but is considered the "zero" value when an invalid value type tries to be converted to timestamp.

As seen by issue #196 under a new deployment the database table fails to create. After looking over the code it seems like the value of '0000-00-00 00:00:00' is used like a null type.

This fix changes the default for timestop to use null and then the queries to look for 'is null' instead. This allows the table to create and function as expected.

No changes to the gui were needed as it already looks for records with "None" or null and converts them in display to '0000-00-00 00:00:00'.

I'm up for doing this different, this way just seems to work from what I can tell in the code.