Fix for zero valued int and float attributes in core.event._eventTypeClassFactory()

Zero valued int and float attributes belonging to an `_eventTypeClassFactory()` object are tested as `False`.
This creates problems like this one:
```python
>>> from obspy.core.event import CompositeTime
>>> c = CompositeTime(2011, 1, 1, 0, 0, 0)
>>> print c
CompositeTime(year=2011, month=1, day=1)
```
where `hour`, `minute` and `second` are not printed, because equal to `0`.

Or this one:
```python
>>> from obspy.core.event import CompositeTime
>>> c = CompositeTime()
>>> c.hour = 0
>>> print bool(c)
False
```
where `c` tests as False, even if not empty.

This pull request fixes this behavior and produces the correct output, i.e.:
```python
CompositeTime
	   year: 2011
	  month: 1
	    day: 1
	   hour: 0
	 minute: 0
	 second: 0.0
```
for the first example, and:
```python
True
```
for the second one.

Tests related to this pull request are all passing.
However, not all the tests are passing right now ~~(and the Travis build should likely fail) due to unrelated modifications to other parts of the codebase (`obspy.imaging`).~~ (edit: merged master with fixed image comparison tests)