Points now default to the empty text.
Refactored Interval and Point.
Fixer a docstring typo.
Replaced some map functions with list comprehensions.
Cleaned up class TextGrid.

* Added function delete_tier.
* Renamed function del_tiers to delete_tiers.
* Cleaned up docstrings.
Refactored Tier, IntervalTier and PointTier.

- Methods from IntervalTier and PointTier moved to Tier.
- Rewrote and extended get_nearest_object in Tier.
- Moved shift_boundaries to section "High level functions".
- Rewrote docstrings.
Fixed a bug in AnnotationObject.__eq__.
Fixed docstrings in AnnotationObject.
Added test cases for Tier.
Missed a function call when renaming.
Finalised function Tier._get_nearest_object().

* Now returns a set of objects.
* Unit test.
* Added a note on its behaviour when time coincides with a boundary.
Removed remaining map and filter use.
Reading and writing files adapted to changes.

* In addition to end times, start times are now also corrected.
* Gaps in IntervalTiers are filled with empty intervals when writing.
* It is now possible (actually the default behaviour) to read files and ignore empty intervals.
* Unit tests for gap filling.