v.0.5.0

Multiple sheet reading is now supported:
- The `Sheets()` method lets you retrieve a list of all sheets present
in the file.
`ChangeSheet($Index)` method changes the sheet in the reader to the one
specified.

Previously temporary files that were extracted, were deleted after the
SpreadsheetReader was destroyed but the empty directories remained. Now
those are cleaned up as well.

Added CHANGELOG, changed README, bumped version number, added
formatting and clarifiation for license.