Fix behavior of `to_offset` with leading zeroes

Currently, `pandas.tseries.frequencies.to_offset` erroneously returns a zero time offset when the first part of its argument has a numerical value of zero, even if later parts have nonzero values.  For instance,

    In [123]: pandas.tseries.frequencies.to_offset('00H 00T 01S')
    Out[123]: <0 * Days>

In this patch the sign check is applied before conversion to `int` in order to support offsets like `'-00H 00T 01S'`.
TST: Add tests for `to_offset` with leading zeroes

Test for the issues fixed by cbc8c73fdf617a4644baa130dff05476c7a61645.
Add `to_offset` fix to release notes
Merge pull request #6391 from ischwabacher/patch-1

BUG: Fix behavior of to_offset with leading zeroes