Implements str iterator
Add test for basic builtin types.
Add fatfs readme/license.
Move user switch code into a separate module

* Move user switch code from main.c into a separate module (usrsw)
* Add usrsw.c to Makefile
Merge pull request #78 from chipaca/list_index

Implements list.index. Fixes issue #57.
Fix qstr in objlist.c; add more tests for list.index.

list.index fails on an edge case.
Merge pull request #80 from xyb/striter

Implements str iterator
Merge pull request #82 from iabdalkader/master

Move user switch code into a separate module
stm: rename sw_xx to switch_xx; change Python bindings to new version.
stm: allow more flash for the binary.
Fix off-by-one in non-default values of index's 2nd and 3rd arguments.