* Add helper function/decorator for deprecation warning

This logs warnings about deprecated functions and also shows the
filename as well as the linenumber.

* enhance documentation

* try to fix python3 (blindly)

* deprecation: use one function to log and one function as decorator

* fix linting

* fix typo `asypte` and remove casting error

* add clipping

* LMDB fix

LMDB was slow (>35min for ILSVRC2012_img_train.lmdb) when iterating
over the entries to find the keys. Now, it is possible to specify
the key_format, such that there is no overhead anymore.

Changes:
- use key_format to generate keys
- remove lmdb_open in reset-state (was open db twice)
- msgpack raised an exception, which is now catched

* use six to check stringtype

* rename key_format argument in lmdb and proper documentation

* small changes for prefetching

* some docs