Worked on list.pop:
* Fixes issue #51
* Adds a specific error message for when you try to pop an empty list.
* Releases some memory if the list has shurnk a lot.
Merge remote-tracking branch 'upstream/master'
renamed some tests to have better names.
Merge pull request #56 from pfalcon/int_fmt-global

Move INT_FMT, etc. declaration into global mpconfig.h .
Merge pull request #52 from chipaca/master

Tweaks to list.pop.
Add basic implementation of slice object.

So far, only start and stop integer indexes are supported. Step is
not supported, as well as objects of arbitrary types.
Implement BUILD_SLICE opcode (2-arg version).
str: Initial implementation of string slicing.

Only step=1 and non-negative indexes are supported so far.
Enable slice support in config.
slice: Implement special handling of omitted start/stop indexes.
str: Handle non-positive slice indexes.
Add test for byte string slicing.
slice: Allow building with MICROPY_ENABLE_SLICE=0.
str: Throw TypeError for invalid index type and clean up comments.