str slice: Trim slice indexes to be in range.
Fix issue #62: Cache loses data

Use the storage cache not only for writing but also for reading. This
avoids reading stale data and thus data loss.
Merge pull request #60 from chipaca/list_copy

Implemented list.copy. Fixes issue #54.
Merge pull request #64 from pfalcon/str-slice-range-check

str slice: Trim slice indexes to be in range.
Merge git://github.com/stevie67/micropython into stevie67-master
stm: Add comments for storage read from cache.
Merge branch 'master' of github.com:dpgeorge/micropython
Merge remote-tracking branch 'upstream/master' into list_count