initial write_cache implementation
unittest for write_cache
fix typo in write_cache
write to and load from cache

Pyckle cache is built on top of pickle module. It's easy to implement,
however pickle can contain any arbitrary code, which does not expose any
limits implemented in pyckle itself. Therefor never try to load cache
file from unknown source!

It implements the same naming scheme as Python, so for PEP 3147
compliant interpreter cache is stored in
__pycache__/foo.${GET_TAG}.cache, otherwise in the same directory, just
with a suffix .cache.

It does have own header similar, but incompatible with python compiled
files. It contains three 8B long entries, magic sequence
'PYCKLE\x00\x00', and timestamp and size of the original file. The load
function then compare both and return None if does not match.
test load from cache as well
more cache testcases
read_cache: raises exception on mismatch

raising of an exception is better, because None is valid (but not that
probably) return valud of pyckle document. It also enhances a test
quality, so tests were adapted to check the specific error messages.

Fixed few comments
dump/load caching support

see $SUBJ - it does even include unittests
fix doc strings of main functions

let use the same style in such tiny library
(c)python2 compatibility

* skipped performance test as the expected gain on python 2.7 is ten
times lower than one I got for 3.3
Merge branch 'cache'

Conflicts:
	pyckle/__init__.py
pypy2 compatibility

The difference between various approaches towards getting attributes
from builtins/__builtins__ seems to be funny.
add changelog, raise to 1.93, add classifiers