Search extra patches from given directory then apply it during build (#91)

```
$ env PYTHON_PATCH_PATH="./patches" PYTHON_PATCH_OPTS="-p1" pyenv install 2.7.6
```
Update README about `PYTHON_PATCH_PATH`.
Use `cp -RPp` to copy share directories recursively.

We have some patches for CPython in `share/python-build/patches`.
Merge branch 'apply-extra-patches'