DEPR: pandas.stats.var, pandas.stats.plm, pandas.stats.ols, pandas.stats.fama_macbeth
closes #6077 

this cleans ups the remainder of the `pandas.stats` modules

we are directing:
- `OLS` -> `http://statsmodels.sourceforge.net/stable/regression.html`
- `PanelOLS/MovingOLS/PooledOLS` to `http://statsmodels.sourceforge.net/stable/mixed_linear.html`
- `VAR` to `http://statsmodels.sourceforge.net/stable/vector_ar.html#var`
- `fama_macbeth` to `http://statsmodels.sourceforge.net/stable/index.html` (as not implemented in statsmodell, but good place to start)
