improve mechanism by which to find parallel HDF5

So far, CMakeLists.txt made the assumption that ${HDF5_LIB} is always defined
in order to determine if it has parallel support. This is not the case, compare
with bug #25.
Make use of FIND_PACKAGE(HDF5) now; it sets HDF5_IS_PARALLEL anyways.
remove tabs from CMakeLists.txt
Merge branch 'cmake-hdf5' of https://github.com/nschloe/netcdf-c into nschloe-cmake-hdf5
Merge branch 'master' of https://github.com/Unidata/netcdf-c