refactor checking clevel
check clevel in pack_array too
refactor checking the type of bytesobj
refactor checking length of input
refactor checking type of address
group argument checks together
check typesize too

This is important, because blosc may segfault if the typesize is zero.
throw in some pep8 fixes for good measure
Merge pull request #43 from esc/refactor_arg_checking

Fixes #29: refactor type and value checking