TS map calculator update and docs

Modified command line tool to work with different pixels sizes. Added some documentation and brief tutorial.
Another error from the command line tool on Python 3:
```
$ gammapy-ts-image all.fits.gz --threshold 0 --scale 0
WARNING: AstropyDeprecationWarning: Use of an 'eval' method when defining subclasses of FittableModel is deprecated; please rename this method to 'evaluate'.  Otherwise its semantics remain the same. [astropy.modeling.core]
WARNING - AstropyDeprecationWarning: Use of an 'eval' method when defining subclasses of FittableModel is deprecated; please rename this method to 'evaluate'.  Otherwise its semantics remain the same.
INFO - Reading all.fits.gz
INFO - Computing (residual) TS map for scale 0.0 deg
INFO - Using down sampling factor of 1
INFO - No down sampling used.
INFO - Using correlation radius of 6.5 pix to estimate initial flux.
INFO - Using 8 cores to compute TS map.
INFO - Writing ts_map_0.00.fits
Traceback (most recent call last):
  File "/Users/deil/Library/Python/3.4/bin/gammapy-ts-image", line 4, in <module>
    __import__('pkg_resources').run_script('gammapy==0.2.dev1086', 'gammapy-ts-image')
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/pkg_resources.py", line 517, in run_script
    self.require(requires)[0].run_script(script_name, ns)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/pkg_resources.py", line 1436, in run_script
    exec(code, namespace, namespace)
  File "/Users/deil/Library/Python/3.4/lib/python/site-packages/gammapy-0.2.dev1086-py3.4.egg/EGG-INFO/scripts/gammapy-ts-image", line 123, in <module>
    dict_to_hdulist(results, maps['ExpGammaMap'].header).writeto(filename, clobber=args.clobber)
  File "/Users/deil/Library/Python/3.4/lib/python/site-packages/gammapy-0.2.dev1086-py3.4.egg/gammapy/image/utils.py", line 391, in dict_to_hdulist
    for name, image in image_dict.iteritems():
AttributeError: 'OrderedDict' object has no attribute 'iteritems'
```

You can get a simultaneous Python 2 and 3 installation from apt-get or conda for local testing.