Added [develop] extra to setup.py to install development dependencies

Use

    pip install -e .[develop]

 to prime your environment for development, this will install all requirements both for running as well as for development tasks (such as running unit tests or compiling the documentation) and register OctoPrint as an editable python package as well.

setup.py

@@ -288,6 +288,9 @@ def params():
 	include_package_data = True
 	zip_safe = False
 	install_requires = open("requirements.txt").read().split("\n")
+	extras_require = dict(
+		develop=open("requirements-dev.txt").read().split("\n")
+	)
 
 	entry_points = {
 		"console_scripts": [