setup.py: prepare for pip

[fnrizzi]

@kennychowdhary
some specific things to figure out:

• where to put the init.py?
• what else to add to setup.py?
• more checks to do inside setup.py: e.g. cmake version and if CXX is found in environemtn with maybe more detailed message as to what version to use ideally
• what does install do? does it copy just the library? if so, what other "regular" files we need to tell setup.py to install?
  e.g. licence, etc
• if I do python setup.py build I know the build directory is created inside the pressio4py repo. Is. there a way to put the build directory outside the source? I don't like to have builds inside source tree.
• also, if I do pip install pressio4py, where is the build directory created?