Fix issue 34 (SFTPFile prefetch assumes response order matches requests)

SFTPFile._async_response gets a new 'num' parameter giving the request
number.  This can be matched up with the return value of
SFTPClient._async_request() to retrieve data specific to that request.

The prefetch queue SFTPFile._prefetch_reads is replaced with the dict
_prefetch_extents, which maps request numbers to (offset,length)
tuples.

A lock is used to exclude the case where a response arrives in
_async_response before _prefetch_thread has updated it.
Import paramiko.org repository @ 3ac370054ef10fb060fe75fff25fe3a70ecc02c0
Meta prep work (reqs.txt, tasks.py)
Start fleshing out two-site setup
Use new behavior from newer Invoke
Rename 'main' doctree to 'www'; refactor sphinx task conf
Blog only used/exists in www, don't import it in docs site
Migrate 1.10.x NEWS entries to Sphinx changelog
Nuke extraneous colons, add 'major' notes
Set up Intersphinx so www can ref docs
Tweak logo settings (& rely on new alabaster defaults)
Update to new alabaster-driven nav sidebar
Don't display blog for now
Bump Invoke requirement to newly released version w/ better Collection.from_module
Add sites/docs building to Travis
Stricter/more useful doc builds in Travis
Prevent warning about unlinked blog page
Move API doc sister link to www site only, derp
Comment out intersphinx pending #256
Merge branch '1.10' into 1.11

Conflicts:
	dev-requirements.txt
	tox.ini