Add webassets project

Webassets enables SCSS compilation and asset minification
The pelican-assets plugin was added to integrate it into Pelican
diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..9d47b3e
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1,6 @@
+# Update when 0.11 is released (needed for slimit and libsass support)
+webassets==dev
+
+cssmin==0.2.0
+libsass==0.7.0
+slimit==0.8.1