Python: Fix RPATH and add source installation instructions

[asfimport]

While Arrow is in development, users may install libarrow in locations other than /usr/local (especially building both release and debug builds) – for this to work without LD_LIBRARY_PATH additions the rpath needs to be configured.

As part of this I'll add basic installation instructions for Python users on Linux and OS X.

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-53. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Well, this has ended up being a massive time sink. I'll have to appeal to OS X rpath experts to be able to package and install both Arrow C++ and the Python extensions in a portable way.

[asfimport]

Wes McKinney / @wesm:
I was able to resolve this essentially through permutation testing (seriously) in #23. rpaths are confusing