ENH: Series.idxnonzero() to return the index of nonzero entries
Added a straightforward Serie function '`idxnonzero`', similar to `idxmin`/`idxmax`

When filtering data, a `nonzero` pattern is actually very handy; yet panda’s version returns the iloc (for compatibility with numpy), which prevent to use the result on a differently-shaped dataframe, defeating the power of pandas indexing.

This simple patch propose a version of nonzero which returns an actual pandas Index.
