Fix bug with full bins in Fourier transforms

I also added a test to make sure the time mesh is twice as
long as the frequency mesh. Obviously now some tests don't
pass... I will fix them in the next commit.
Fix wrong minus sign in FT definition

This is a copy of commit 3fcf727 that has been done on the master branch.
python wrapper. Pre/postcall can now be inlined

- Add the possibility to give a function "on the fly"
  for the precall and postcall of a python wrapped functoin.
- No change for previous code, it is a simple new feature.
- changed test accordingly. See my_module_desc.py for an example.
Fix #98

- A silly copy paste error : the operator - for matrix expression was
  defined twice.
- second error : a typo in unary method.
Cmake. Fix #97 and #89

- C++14 mode not automatic. Was based on compiler version,
  but the version of the C++ lib also matters....
  Now, there is an explicit USE_CPP14 option [default =OFF]
  to set to compile in c++1y mode.
  Solve also #89, i.e. the flags is now in TRIQS_CXX_DEFINITIONS and
  therefore pass to applications (including ipython magic).

- Python_use_mpi4py is now ON by default.
  Rational : it has been used for a while by e.g. Thomas,
  and it is necessary on OS X (boost.mpi.python raises a lot of issues).
  Hence we put it as default.

- Rename the option Build_Triqs_General_Tools_Test to a simpler
  Build_Tests
Fix make_mesh_fourier_compatible to correct size

The number of points in imaginary time must be at least twice
as big as the number of pionts in matsubara frequencies.
Update tests related to Fourier transforms
Fix #101 operator: add python wrapper for is_zero

- added is_zero in python
- reorder a bit (module defined first), but that is cosmetics
fourier. Add one overload to make_gf_from_fourier API.

- To be able to give explicitely the number of point.
- two functions : if the number of points is not given, take as before.
- updated the doc.
Fix #100.

- The workaround for align_t in C++14 mode has been fixed in recent
  version of clang.
- Does not apply to OS X (it uses libc++).
updated installation instructions for Mac OS X
Draft libclang based python wrapper desc generator

- Given a C++ file, e.g. a class,
  it calls libclang to parse the C++, and retrieve from
  its AST the necessary info to write a xxx_desc.py file.

- THIS IS WORK IN PROGRESS. There are several corner cases for which we
  may want (or not) the script to do better.
- It is not designed to be used automatically, but to to 90 % of the
  boring typesetting work...
- The preamble still needs manual choices

- The properties, methods, functions are automatically declared in
  the _desc file, in the simplest possible way.
- An option --properties, -p : to transform some simple methods or
  get_x, set_x into python properties, not methods.
  Cf doc.

- requires clang (tested on 3.4).
- the script is configured by cmake and installed in
  INSTALLATION_DIRECTORY/bin, with some other files.
  It can only be used for applications, after the lib has been installed.
  It is cmake configured, to include automatically the various include
  paths configure in the triqs installation, including the triqs install dir
  in order to simplify invocation.

- TODO : improve, and test more in real cases.
hdfarchive + gf : two corrections

- hdfarchive : transform the key to string with str
  because they can be unicode and C wrapper does not convert python unicode strings.
- gf : Correct the scheme for BlockGf : not very clean, to be improved ?
Add indicesR/L to gf_desc
fixed cmake dependency warnings
added check for mpi4py in config
minor change in install doc for osx
Prettification.
More prettification: can not --> cannot
Doc changes -- installation