BUGFIX: Cleaned up the creation and saving of textgrids and tiers

Before, textgrids could be opened with silences removed (which would
make a textgrid with no entries unopenable) and textgrids could be saved
with 'gaps' in the textgrid (which would lead to strange behavior
in praat, such as having unselectable regions of the textgrid).
Both of these possibilities have been removed.
REFACTOR: Pulled out the overlap check code so everyone can use it
FEATURE: Added several new functions for manipulating intervals
FEATURE: Code to merge tiers into super one super tier
REFACTOR: Renamed project, import method

# Before

from pyPraat import praatIO

# Now

import praatio
FEATURE: Enforce textgrid wellformedness
FEATURE: Expose append and sort functionality to textgrids

This feature already exists for tiers, but now it is possible
to do for textgrids as well.
FEATURE: Added test code for mergeTiers()

Also brought the other examples in line with the recent
changes to praatio