BUG: History no longer fails on length-1 '1m' price-only HistorySpecs

The bug occurred because there is a special case in the initial window setup
code for handling the case where only a length-1 history is used for a given
frequency.  Previously, the code was incorrectly calculating the period end
using a hard-coded expression for the end of the day (the correct behavior for
a length-1 '1d' history), and then using the frequency object to calculate the
period start for the window.  In the case of length 1 '1m' data, this resulted
in an initial window whose start and end was the last minute of the day rather
than the first minute of the day.  For non-price fields, this error doesn't
matter, because the window is only used for rolling digests (which doesn't
happen when there's only a length-1 history), and for the forward-filling logic
(which only happens on price fields).  For a length-1 '1m' price, however, the
incorrect window causes us to attempt to forward-fill an empty panel, resulting
in an IndexError when we do an iloc[0] on a length-0 axis.
PERF: Replace .ix usages with with .loc in TradingEnvironment.

Replace usage of .ix in TradingEnvironment with .loc when we know that we're
using an index key.

DataFrame.ix can be used with either integer or key-based indices, and as such
it incurs an overhead for figuring out which you meant.
MAINT: One way to set sim_params and data_frequency.

There were sevaral places you could supply sim_params
in TradingAlgorithm (__init__, run). This got confusing
as its not clear who updated what and which one was the
correct one to use at each time.

Then there were to ways to define data_frequency, one in
__init__() and one in the sim_params which also added code
complexity.

This refactor makes it explicit that sim_params are to be
passed to __init__() only. Moreover, data_frequency is
only stored in sim_params. For backwards compatibility,
it can still be supplied separately but will link to
the one in sim_params.

For example, you could create new sim params via:

sim_params = create_simulation_parameters(data_frequency='minute')
algo = MyAlgo(sim_params)
algo.run(data)

In addition, perf_tracker only gets initialized in one place:
_create_generator() which should also make the various ways
of running an algorithm more deterministic.

This also fixes a bug with SimulationParameters where
you could not change the period_start. Unfortunately, the
current implementation still requieres an implicit call to
update the internal variables.
MAINT: Remove unused annualizer code.
ENH: Change open to midnight if using daily input data.