setup.py: Add LIBGIT2_LIB to override the library search path

I build libgit2 in a `build/` subdirectory of a Git checkout, and
never bother installing it.  I'd like to link pygit2 against it there,
but LIBGIT2 assumes libgit2.so will be in the default library search
path or $LIBGIT/lib.  This patch adds LIBGIT2_LIB, a new environment
variable for overriding the additional library search paths.

An alternative to this patch is to set LDFLAGS="-L..." explicitly
before building pygit2.  I've avoided that approach here minimize the
difference between configuring this library path explicitly, and
configuring implicitly via LIBGIT2.
README.rst: document non-standard linking on *nix

There have been a few comments on pygit2's issue tracker about linking
problems on Unix/Linux [1].  David suggested that a README comment
might be in order, so here it is.

I think that people new to *nix linking should probably just stick to
installing to standard locations, since a short README blurb is never
going to do justice to a subject that probably deserves it's own
chapter.  Instead of explaining what's going on, I just give a quick
recipe for building and installing libgit2 under /usr/local (the
default location), and then building pygit2 against that libgit2
(using RUNPATH).

[1]: https://github.com/libgit2/pygit2/issues/134
Update to libgit2 changes in git_diff_* functions

See libgit2's 56c72b759c3ad commit
Correct rst syntax errors in README.rst
added pygit api link
Merge remote-tracking branch 'cholin/added_api_link'
Implement Iterator protocol on Repository

Uses a stupid implementation that builds a list in memory and returns
it's iterator.
Ensure that odb is always freed
Handle errors while building the list gracefully
Don't leave dangling references to oid strings
Always decref the new string. Don't set err.
Shamelessly add richo to AUTHORS
Use correct prototypes
return NULL over -1
Merge remote-tracking branch 'richo/bugs/compiler_warnings'
Make Repostirory_as_iter shorter

Error_set already returns NULL
Readme: add table of contents
sphinx-quickstart
Use autodocs
Copy docs from the readme file