update get_store() calls to pandas 0.13 API
args are no longer accepted by get_store(), only kwargs
fix internal DatetimeIndex conversion to use pandas.to_datetime()
it's more idiomatic pandas and 0.13 enforces it
drop support for pandas 0.10.1
there are some odd tests that fail only on 0.10.1 and I don't have much time at
the moment to look into it - so I'm going to punt on it :/
TST: fix ghcn_daily_test so it works with pandas 0.11
by casting explicitly to a daily period instead of implictly with date string
add note in changelog about dropping 0.10.1 support and adding 0.13