Clean up Makefile and requirements.txt [ci skip]

- Remove relative calls to pip, python (assume virtualenv activated or otherwise "safe" global env)
- Full define development and documentation requirements in requirements.txt

``make test-dep`` installs documentation dependencies too, hopefully that's not too annoying.

This means development process for documentation and code can begin with:

    virtualenv .
    source bin/activate
    pip install -r requirements.txt

Makefile

@@ -14,12 +14,12 @@ help:
 
 pre:
 	virtualenv .
-	bin/pip install -r requirements.txt
-	bin/python setup.py develop
-	bin/python selftest.py
-	bin/nosetests Tests/test_*.py
-	bin/python setup.py install
-	bin/python test-installed.py
+	pip install -r requirements.txt
+	python setup.py develop
+	python selftest.py
+	nosetests Tests/test_*.py
+	python setup.py install
+	python test-installed.py
 	check-manifest
 	pyroma .
 	viewdoc
@@ -52,7 +52,7 @@ coverage:
 	coverage report
 
 test-dep:
-	pip install coveralls nose nose-cov pep8 pyflakes
+	pip install -r requirements.txt
 
 docs:
 	$(MAKE) -C docs html

requirements.txt

@@ -1,3 +1,8 @@
-# Testing reqs
+# Testing and documentation requirements
 -e .
+-r docs/requirements.txt
+coveralls
 nose
+nose-cov
+pep8
+pyflakes