Stream/Trace.plot() improvements

Improve plotting:

 - better x-tick formatting
   - ticks get updated automatically
   - ticks are now correct (float seconds were cut off previously and tick locations were fixed rather than aligned on even values)
   - format of tick labels gets set automatically depending on distance between ticks (dynamically!)
 - share x axes of subplots (for interactive zooming)

Speed of plotting is not decreased.

Here is a script to run some plots:
```python
from obspy import read
tr = read()[0]
for type in (None, "relative"):
    for method in ("full", "minmax"):
        for samp_rate in [0.0001, 0.001, 0.1, 1, 100, 10000]:
            print type, method, samp_rate
            tr.stats.sampling_rate = samp_rate
            tr.plot(type=type, method=method)

Y tick labels should also be reworked..
```