improved semantics for apply_remap
no reason for these to be methods
make source object mutation optional
change semantics of apply_filter; filter _after_ mapping
hoist all_or_nothing out of the loop
remove *skip* special case: just omit the key
improved code style and commentary
string keys need quotes in python
PEP8
Update README.markdown

To reflect updated semantics
Update README.markdown

to fix an editorial error: s/<postproc>.json/<postproc>.py/
require .py file extension for "postproc" filter
correct a syntax error

I introduced a syntax error during refactoring. Not sure why PEP8 didn't notice it.
need to set the objectID on the filtered doc
better use of variables
remove pointless comment
better readability for dictionary literals
document `apply_filter` and `apply_remap`
remove a function that is never used
need to postproc against `filtered_doc`