posterdown_memoir phased to posterdown_pdf

Fully-ish customised sizing of the poster and colours :partly_sunny:
diff --git a/NAMESPACE b/NAMESPACE
index 462c2e1..448b9ce 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -1,6 +1,6 @@
 # Generated by roxygen2: do not edit by hand
 
-export(posterdown_memoir)
+export(posterdown_pdf)
 import(devtools)
 import(dplyr)
 import(ggplot2)