docfixes in Variables section of templates.rst

I was distracted by "alternative" -> alternatively, then changed other bits and pieces in the section.
fixes suggested by @Naddiseo, and clarify that "what a variable looks like" really means "what attributes a variable has."
force a reload of the template if bytecode fails to unmarshall.  issue #281
explicitly catch marshal errors rather than genertic exception #281
Spelling, grammar, and punctuation fixes to extensions docs.
Fixed typo (igored -> ignored)
fixed some typos and clarifying
Fix typo.
Fixed typo
Fix loop context length calculation for iterators. Fixes #244

This resulted in wrong values for revindex and revindex0
while looping over an iterator.
Improved performance of filters.do_truncate()
Edit code comments for readability

Changed a few things to make it more understandable
Switch to new server
Update Jinja2 tip link
fix os.name of Windows

os.name of Windows is 'nt', https://docs.python.org/3.4/library/os.html#os.name
use abs path for cache key
Update environment.py
Update environment.py
if template is not file, use name for cache key
close temp file

The test can't remove the temp file on Windows, because `mkstemp()` will open the temp.
check for if loader does not implement get_source()