Extend syntax and allow to set relative due dates

[Stefan Glase]

Dates shall be set relative to the current date or relative to the date of the parent issue.

Tasks

• allow to specify a due date relative to @now plus/minus years/months/weeks/days
• allow to specify a due date relative to @inherit date from parent plus/minus years/months/weeks/days
• extend documentation
• test for dueDate and custom field

Examples

- due in 5 days
  dueDate: @now+5d
- due in one month and 4 days
  dueDate: @now+4d1m
- due one day before parent
  dueDate: @inherit-1d
- due now
  dueDate: @now
- due as parent
  dueDate: @inherit
- due with spaces
  dueDate: @now + 1y

[Stefan Glase]

Also allow for @now and @inherit without any calculations on it:

@Test
public void returnDateForRelativeDateNow() {
    Timestamp timestamp = dateTimeStringService.dateStringToTimestamp("@now", null);
    assertThat(timestamp.toString(), is(equalTo(todayAsString(0, 0, 0, 0))));
}

@Test
public void returnDateForRelativeDateInherit() {
    Timestamp timestamp = dateTimeStringService.dateStringToTimestamp("@inherit", new Timestamp(new Date().getTime()));
    assertThat(timestamp.toString(), is(equalTo(todayAsString(0, 0, 0, 0))));
}