Python 3 patches. Should now work in Python 2 and 3
Merge remote-tracking branch 'bempp/python3' into rsdt/python3

* bempp/python3:
  Python 3 patches. Should now work in Python 2 and 3

* Added further python 3 patches

Conflicts:
	python/bempp/assembly/boundary_operator.mako.pyx
	python/bempp/assembly/context.mako.pyx
	python/bempp/space/space.mako.pxd
	python/bempp/space/space.mako.pyx
Removing NestedAttributes, since it is used no longuer
Removed nested_attributes.py from CMakeLists.txt
TBB can be defined via environment variables
Merge branch 'rsdt/python3' of github.com:bempp/bempp into rsdt/python3

* 'rsdt/python3' of github.com:bempp/bempp:
  Removed nested_attributes.py from CMakeLists.txt
Requiring cython >= 0.21
Requiring cython>=0.21 through pip
Dune to build from pre-installs as well as lookup

Closes #169

Also makes dependency on external libraries more explicit in exported package.
Syntax error introduced in d985eb77706229a65e927f8

Was setting correct hint for path to dune prefix from environment variable.
Incorrect path capitalization
Better way to find duneproject program

Added hints as to its location, based on dune-common component.
Also, make sure that local dune hacks are called only once all dune external
stuff are installed.
Intel compiler need some constructors made explicit

Intel requires the constructor of classes derived from
boost::static_visitor<something> to be called explicitly. Not sure whether this
is a boost or intel bug/feature.
Simplified FindDune functionality
Dune local project always included

Otherwise, requires to many cmake recursive calls?
Export user-specified CXXFLAGS

Flags that are given by the user when compiling BEM++ are exported in
BemppConfig.cmake.
Replace cython template with explicit overloads

Cython in debug mode does not handle templating too well. So replace some
templating with explicit overloads.
Reworked boundary operator operations to avoid throwing in C++

Somehow, it would create incorrect calls to python's free on jenkins macs. No
idea why.
Merge pull request #166 from bempp/rsdt/python3

Rsdt/python3