v0.5.0: Remove special case for Time and DateTime attribute types

To fix #6, this release removes the special handling of Date and DateTime attribute types. Until now, these were supposed to be parsed into Date or DateTime objects respectively, but this was problematic as Insight uses Jira's Date format for the API instead of a standard format. Until this can be worked around, the library will simply return the date as a string, so the user can parse that as needed.