Issue/normalize args

Addresses #279, and starts to address #252. Will address #252 before merge.

#117 is addressed implicitly for any scripts using these functions (which should be all...)
TODO:
- [x] Unify and standardize argument parsing
- [x] Update scripts to use standardized arg parsing
- [x] Add a function pull ksize, nhashes, and hashsize from saved hash
- [x] Add standardized checkpointing args (#252)
- [x] Tests

The new functions are just python methods on the khmer module, which read the first couple bytes of the hash and extract the values. The custom action for the loadhash parameter uses this to override the user values if they are given for k, x, and n. If the user supplies any, they are warned that they will be overwritten; the warning is suppressed when the defaults are used (ie no user parameters given).

EDIT: In the interest of getting this merged in, I'm forgoing standardized checkpointing args for now. The current patch just adds a flag to normalize-by-median, as noted in #252.