Added support of non-default cdfMod in test script since scipy seems to bee broken
added calls to rm to leave a clean file system behind after running the test suite
 * Added named constants `CDF_MOD_*` for available cdf mods
 * import dataset class of cdf mod to Cdo().cdf instead of whole package
 * loadCdf() is called on first access to Cdo().cdf to initialize it.
Use cdo.CDF_MOD_NETCDF4 to set cdfMod
Fix occurence of segmentation fault when using returnArray (https://github.com/Try2Code/cdo-bindings/issues/4) with cdfMod=CDF_MOD_SCIPY and scipy v0.14.
However, when using returnCdf together with an direct access to variable data, like `cdo.stdatm("0",options="-f nc",returnCdf=True).variables["T"][:]` there will
still be an segmentation fault due to a bug in scipy.io.netcdf.netcdf_file (see https://github.com/scipy/scipy/issues/3630). In this case one will have to store the filehandle and then access the data:
`cdf = cdo.stdatm("0",options="-f nc",returnCdf=True); data = cdf.variables["T"][:]`
Merge pull request #5 from martinclaus/master

fixed test_cdo and segmentation fault using returnArray