BUG: 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>

This change checks the sign of the numeric part of the argument before converting to `int` in order to support offsets like `'-00H 00T 01S'`.