Merge pull request #43 from jhermann/fix-direct-local-symlink

Fix "local" symlink in cases where it points directly to its containing directory
ignore local virtualenv dirs
Run main package install and tests in source directory
Merge pull request #47 from jhermann/run-install-and-tests-in-sourcedir

Run install and tests in sourcedir
Ignore paths in debian/ generated by package building
Merge pull request #49 from jhermann/ignore-buildpackage-files

Ignore paths in debian/ generated by package building
Remove commands that might run system's Python

These are dh_auto_build and dh_auto_clean, which don't make any sense
when we are running commands inside the virtual environment anyway.

Closes GH-51
Merge pull request #52 from nailor/avoid-system-python

Remove commands that might run system's Python
Update usage.rst

Documenting an "undocumented feature" ;)
Merge pull request #54 from schainker-digbil/patch-1

Update usage.rst
Allow user to pass in extra pip parameters
Adding in a test
Docs update
Code review changes from PR/58
Changes from a cr of PR/58
Merge pull request #58 from labeneator/master

Allow for custom pip flags to be passed to the pip command
Change behavior by spliting build / install / clean phases.

/!\ Breaks compatibility

```
overrides_dh_virtualenv:
	dh_virtualenv (options)
```
become
```
overrides_dh_virtualenv_build:
	dh_virtualenv_build (options for build)

overrides_dh_virtualenv_install:
	dh_virtualenv_install (options for install)

overrides_dh_virtualenv_clean:
	dh_virtualenv_clean (there is no option ;))
```

On a typical run:
* `dh_virtualenv_build` create a virtualenv in `build_dir` (default to: `debian/dh_virtualenv`) and a `debian/dh_virtualenv.debhelper` file to log `build_dir` path.
* `dh_virtualenv_install` copy file from `build_dir` to correct place in package
* `dh_virtualenv_clean` clean `build_dir` and log file.

Additional option:
We can set destination dir in `dh_virtualenv_install` to install venv in another path in final package (default remain in `/usr/share/python/PACKAGE`)
Update README.md