Unit tests fail because of Sherpa import

@zblz The `gammafit` unit tests currently fail because of this:
https://travis-ci.org/zblz/gammafit#L335
```
Stderr:
/home/travis/build/zblz/gammafit/build/lib.linux-x86_64-2.7/gammafit/__init__.py:134: UserWarning: Sherpa modules require 'sherpa'
  warn("Sherpa modules require 'sherpa'")
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/travis/virtualenv/python2.7_with_system_site_packages/local/lib/python2.7/site-packages/astropy/config/configuration.py", line 654, in generate_all_config_items
    for cfgitem in six.itervalues(get_config_items(nm)):
  File "/home/travis/virtualenv/python2.7_with_system_site_packages/local/lib/python2.7/site-packages/astropy/config/configuration.py", line 540, in get_config_items
    __import__(packageormod)
  File "/home/travis/build/zblz/gammafit/build/lib.linux-x86_64-2.7/gammafit/sherpamod.py", line 8, in <module>
    from sherpa.models.parameter import Parameter, tinyval
ImportError: No module named sherpa.models.parameter
```

How should this be fixed?
Of course it's possible to move the imports into the functions and methods, but the `InverseCompton` class also doesn't show up in the [docs](http://gammafit.readthedocs.org/en/latest/search.html?q=InverseCompton), so maybe you want the file ignored altogether and it should simply be moved in some other folder that's not part of `gammafit`?