added tolerance for migration rates just above 1 or just below 0.
fixed out_of_bounds behavior for very small values ou out-of-bounds
added bootstrap support
remove extraneous parentheses
adjust indentation to four spaces, and other spacing adjustments
remove parentheses for print statements

This is Python 2, without the future-import of the print function, so
the print statement has its own special syntax, and isn't a function.
Merge branch 'style'

Conflicts:
	tracts.py

Changes from Simon's updates were incorporated, but the style was
adjusted to match the changes from my the style branch.

The style branch included some renaming of variables and functions,
namely to include underscores at word boundaries. Hopefully in selecting
Simon's versions, inconsistencies in the names were not introduced. To
verify this, either a list of identifiers needs to be compiled, or
testing should be done.