fix(precisedelta): keep sign of negative timedeltas (#379)

[artvepa80]

Fixes #379

Changes proposed in this pull request:

• precisedelta dropped the sign of negative timedeltas and negative numbers of seconds, so they rendered the same as the positive duration.
• Remember the original sign before _date_and_delta() takes the absolute value, and prefix the formatted result with - when the input is negative.
• Add regression tests for negative timedeltas, a negative number of seconds, and the positive and zero cases.