Fix index out-of-range in pos_to_index

Fix a crash occurring from trying to access a list with a negative index when drag-selecting text above the field box boundaries.(cherry picked from commit 609538b441a0dcf7890b4656c98673ee8de8820d)
Merge changes from pull request #149
Pocket Edition: (#212) New textures and block types.

Submodule pymclevel:
    > 307dfcb - Ignore profiling outputs
    > a84bb90 - Change coordinates of purple block texture to a different unused block.
    > 76e990e - MCJavaLevel: Fix AttributeError in saveInPlace
    > 06f7958 - Added: Nether Brick Slabs
    > 9994317 - Cleaned up pocket.yaml
    > 71c9935 - Finished adding new materials (based on wiki and inventory)
    > 66d8c02 - Added some of the new blocks. Nether reactor related still ramaining, and maybe grass.
Updates: Change update URL to bitbucket
Setup: Change url in setup.py
Graphics: Update to 1.4.6 terrain.png

Submodule pymclevel:
    > fda71ca - Update NOTEX for 1.4.6 terrain.png
Fixed: Unicode encode errors in update status callback
Debug: Duplicate UnicodeErrors not having exception names removed
Ignore ENV32, virtualenv for 32-bit python on 64-bit windows
Submodule pymclevel updated.

Submodule pymclevel:
    > c19ba58 - Rename Block 36 to Piston Movement Placeholder
    > 391883f - End Portal Frames are transparent. Fixes #121
    > b2af2d8 - Added block 36
Fixed: Warning message on startup no longer links to GetSatisfaction. The bug is fixed in Minecraft 1.4.
Submodule pymclevel updated.

Submodule pymclevel:
    > 1e649dc - Fixed: File already exists error when deleting and recreating unsavedWorkFolder
Remove debug print from auto-update.
Merge branch 'master' of git://github.com/mcedit/mcedit
Removed some of hardcoded IDs in renderer.py
Removed some more references to alphaMaterials.
Refactored leaf rendering.
Added a separate color for jungle leaves.