Pypi2nix

- fixed a bug with relative paths in requirements files, 68
- ask to report an issue when not known issue is raised, 55
- nicer error message when no source distribution is found, 78
- `pip wheel` errors are now re-raised and caught in python, 77
- using `sha256` instead of `md5`
- *_cache_dir variables not getting set if you specify cache_dir from the cli [Matt Billenstein]
- Add pip_build_dir -- pip was filling up /run on Ubuntu [Matt Billenstein]
- renaming `python.pkgs` -> `python.packages`

- release version detection improved, using pkg_resource to parse version
- buildout support (eg. `pypi2nix -V 2.7 -b buildout.cfg`)
- pip cache is now in pypi2nix cache
- creating `requirements_frozen.txt` alongside `.nix` files. it can be useful for pip/virtualenv
- increased timeout when fetching resources from pypi
- license is now also recognized using classifiers
- output in console is now minimal, `-v/--verbose` added for debugging purposes
- stage1 isnt failing with correct error code. now i'm checking for Error string.
- SSL_CERT_FILE exported in stage1
- `requirements.nix` improved a bit
- improved travis builds

- support for `-e` option, 32
- support for URLs in `requirements.txt`. now it is possible to install Pillow from my fork. All you need to do is put `https://github.com/garbas/Pillow/archive/pkg-config.tar.gz#egg=Pillow` into your `requirements.txt`, 54
- use requests library instead of aiohttp, which fixed `concurrent.futures._base.TimeoutError` errors, 51
- during stage1 LANG is set to `en_US.UTF-8`
- parent python environment was causing recursion

- adding support for darwin platform
- fetching metadata from pypi using aiohttp, 42
- for sources use `sha256` instead of `md5`. when `sha256` is not provided we download package and calculate it, 41
- python version option (`-V`) is now a required option, 39
- `python` "object" now has a `pkgs_top_level` list of all top level packages, 38
- an option `-T` added to turn `doCheck=true;` for all generated packages, 36
- detect more licenses, 29
- multiple `requirements.txt` can now be provided when calling pypi2nix, 34
- run `nix-shell` with `--pure` option

- adding awscli example
- version of deps.nix updated
- updating README
- use `nix-shell` instead of `nix-build` in stage1
- displaying exact command in generated nix files, 21
- travis is building successfully

initial release of reworked pypi2nix