Use decimal values for `coverage`

**NOTE: This change required a small (but important) modification to a few tests!**

## Rationale for change

* Python’s `str.format()` has a built-in mechanism for formatting a decimal value as a percent.
* In `reporting.py`, percents were calculated manually.  This meant that if an attempt was made to use `str.format()`’s built-in percentage format  on `coverage`, we had to divide by 100
* The manually-calculated percentages also had the potential to truncate digits, causing data loss; ideally, the raw data should be preserved until the time comes to format it as a string, at which point the number of digits should be specified in `str.format()`

## Benefits
* `reporting.py` now uses built-in methods to format percentages, which we don’t have to maintain (yay!)
* `coverage` is now consistent within `reporting.py` and values produced in the
* No digits will be truncated; client code can decide how many (or how few) digits to use with `str.format()`
* No need to manually insert a “%” into output strings

## Changed Tests
* This change required modifying a few tests to expect decimal values ranging from `0.000`–`1.000` (instead of `00.0`–`100.0`)
Merge pull request #63 from Zearin/feature/coverage-reporter-percent

#63: Use decimal values for `coverage`
Fixing reporting
Release 1.1.10
Missed some
Release 1.1.11
Prevent non-superscriptable object from exploding

Changing the order of the test for whether or not an error exists stops the
test runner from ending preemptively with a TypeError for object types that
are not superscriptable (for exmple, the Response objects that come from
Flask). This change prevents the TypeError when looking for
test['topic']['error'] but allows the VowAssertionError to bubble up,
including its message about the actual failed assertion while preserving
the related stack trace.
Add tests demonstrating topic-less tests

This exercises print_context with a test that doesn't have a 'topic'
key.
pep8 compliance
don't print an exception message, that's silly.
Merge pull request #64 from sleepynate/patch-1

Fixed #64: Prevent non-superscriptable object from exploding
Renamed files in `tests/assertions/` (remove redundancy)