Factor out cache/munge. Add StoreSession.

The internal cache and munge parts of the global and attr handlers have been
factored out. This allows us to wrap the global and attr handlers and to
short-circuit them, yet still support pre/post cache and munge lookups.

Also added the StoreSession feature which dumps received raw data from
Net-SNMP to a text file in "qual_leaf = var" format. Enabled using
StoreSession param to new(), and filename optionally set with SessionDataFile.

.gitignore

@@ -0,0 +1 @@
+*.db