BUG: mseed, avoid pointer out of bound access

see #678
correcting type in readMSEEDBuffer

and refactoring, see #678
mseed: more verbosity, correcting reclen, tests

It was not possible specifying usefull reclens, trying to correct.

Adding tests related to see #678.

Adding more explicit logging to obspy-readbuffer.c to ease future
debugging, e.g.:
```
python -c "from obspy import read; read('/path/to/test.mseed', verbose=2)"

readMSEEDBuffer(): calling msr_parse with mseed+offset=58461152+0, buflen=8192, reclen=-1, dataflag=0, verbose=2
(null): Unpacking Steim-2 data frames
readMSEEDBuffer(): calling msr_parse with mseed+offset=58461152+4096, buflen=8192, reclen=-1, dataflag=0, verbose=2
(null): Unpacking Steim-2 data frames
```
Merge pull request #678 from mbyt/678

test_full_seed_with_non_default_dataquality failing with strange behavior
avoid some numpy warnings in tests, prevent masking of a tested warning
corrections for test failures

* remove checking for warnings in test_spectrogram. We compare the real
  result, which are the png's, which are correct. I think we should not
  test for other library warnings inside our tests. However, I did not
  trace back, why this warnings dissapeared.
* the test reference data of relcalstack are on the fly corrected for the
  previous buggy arctan call. arctan2 or np.angle need to be called, else
  the quadrant of the tan is not evaluated correctly. However, the
  correction in 2f9876d also removed the conjugate() / imag *= -1.0
  line, which I do not understand. Therefore, I removed it.