CachedSession now caches walk requests and in addtions stores the
received values as get results.
This reduces the number of Get PDUs sent on iteration of tables.

Previously

for i in m.ifDescr.items():
    print(i)

would result in one walk of ifDescr to get the indices and one get
request per value.
Merge pull request #9 from fragfutter/master

CachedSession now caches walk requests...