Use with statement for reading readme file

1. Without file closing it may cause "too many files" error
   on PyPy.  It's complicated to explain, but related to difference
   of memory handling from CPython (reference counting) to
   PyPy (garbage collection).

2. README.md could be missed from distribution.  For this
   now it returns empty string when it faces to IOError.
Use version.txt package_data

1. Installation might cause ImportError because the system didn't
   install requests yet.  itunesiap/__init__.py imports requests.
   setup.py imports itunesiap to get version data.
2. So we move version data from __init__.py to version.txt file.
   Also we use pkg_resources that setuptools (distribute) provides
   to read the file.  Also we use distutils' package_data field and
   add version.txt file to MANIFEST.in list to include the file
   to the distribution.