Add corpusSize function

Resolves #2

This is a bit of a hack, but it does work with old derekovecs servers.

Change-Id: I1c8f81c7341f0df71012f5991bde5edfc83dc2b0
diff --git a/NAMESPACE b/NAMESPACE
index c182313..8a1ecba 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -2,6 +2,7 @@
 
 export("%>%")
 export(collocationScores)
+export(corpusSize)
 export(cosineSimilarity)
 export(countbasedCollocates)
 export(derekovecsApiCall)