doc: added some explanation to AsyncImage to completeness
Further fix for ``lang._handlers`` memory leak related to ``Bubble``.

Original problem: Bubble constructs BubbleContent before finishing its
own construction, thus never initializing its ``_proxy_ref``, causing
``lang.get_proxy`` to return the Bubble itself (this is when creating
handler for BubbleContent's color from parent color). This in turn
caused ``lang._handlers`` to unnecessarily keep it alive. 

The fix: I let ``Widget._proxy_ref`` be generated lazily on demand
instead of relying on ``Widget.__init__``: whenever the user calls
``Widget.proxy_ref`` for the first time, ``Widget._proxy_ref`` will be
initialized. I don't see any particular reason to initialize
``_proxy_ref`` only in the ``__init__`.
Merge remote-tracking branch 'kivy/master' into lang_handler_memory_leak_fix
doc: revision to input/motionevent.py
doc: revisions input/motionevent.py
Merge remote-tracking branch 'kivy/master'
core:text make shorten work with a single word. closes #1334
doc: revisions to sources/installation/installation-windows.rst
doc: revisions to installation/installation-windows.rst
doc: link correction for scroll effect
Update installation-linux.rst

Typo and replaced nightly with daily like the PPA is called.
Re-worked wording or motion events
Merge pull request #1406 from Zen-CODE/motion_event

doc: revisions to input/motionevent.py
Fixing some grammar in the Vector documentation
Fixed a typo in the Vector documentation
Merge pull request #1420 from prtksxna/documentation-typos

Documentation typos
doc: fixed too short underline error on make html
doc: revisions to motionevent.py
doc: revisions to input/postproc/doubletap.py
Fix #1419. Use memoryview instead of deprecated buffer.