ENH:  Allow option to return lower triangular correlation matrix in DataFrame.corr
In a lot of situations it's useful to remove the redundant or trivial (diagonal) entries from a correlation matrix.  For example, if you want to query a correlation matrix to find highly or perfectly correlated columns you have to find some way of manually ignoring the diagonal, or if you want to remove only one of two highly correlated columns, it's useful to be working only with the lower triangular form.

Therefore I think it would be very useful to add a boolean argument to `DataFrame.corr` which specifies if the lower triangular correlation matrix should be returned (if the upper triangular is desired the user can take the transpose) instead of the full.