Mixin block support (`+mixin` calls).

For reference, see original Jade commits ae8e8b7d, cf886ece and 8ec78bba.

Tests seem to pass fine, but I'm not sure about Jinja2 implementation, as
it contains a hack/workaround. Complex nested mixin chains may misbehave.

This commit does not update `visitCodeBlock`/`visitMixin` in `compiler.py`,
as those seem to be unused/legacy code.
Merge pull request #49 from drdaeman/master

Mixin block support (`+mixin` calls)
Merge pull request #50 from thenoviceoof/master

Location (file/line) info in parser exceptions
Improved jinja
Merge branch 'master' of github.com:SyrusAkbary/pyjade
Add support for travis tests
Update README.md
Updated README
Updated version
Update pyjade/ext/tornado/__init__.py

tornado.template is not imported in tornado.__init__, you should import it directly
Merge pull request #54 from reorx/patch-1

Update pyjade/ext/tornado/__init__.py
Fixed #55
Merge branch 'master' of github.com:SyrusAkbary/pyjade
Updated pyjade package version
Fixed convert&tests
Updated test Django example
Added Tornado to readmes
Update pyjade/convert.py

use a more convenient way to involve compilers;
add missing tornado compiler;
fix typo 'aviable';
fix some informal code style

Please test the code after merge it, I did do that because the change was done on web page :)
Passed path filename in include/extend from parser to compiler for modifying it in compilation time.
Fixed #56
Fixed #58