Pycobertura

* a non-zero exit code will be returned if not all changes have been covered. If
`--no-source` is provided then it will only check if coverage has worsened,
which is less strict.

* memoize expensive methods of `Cobertura` (lxml/disk)
* assume source code is UTF-8

* prevent misalignment of source code and line numbers, this would happen when
the source is too long causing it to wrap around.

* pycobertura diff now renders colors in terminal with Python 2.x (worked for
Python 3.x). For this to work we need to require Click 4.0 so that the color
auto-detection of Click can be overridden (not possible in Click 3.0)
* Introduce `Line` namedtuple object which represents a line of source code and
coverage status.
* *BACKWARDS INCOMPATIBLE*: List of tuples generated or handled by various
function now return `Line` objects (namedtuple) for each line.
* add plus sign (+) in front of lines that were added/modified on HTML diff
report
* upgrade to Skeleton 2.0.4 (88f03612b05f093e3f235ced77cf89d3a8fcf846)
* add legend to HTML diff report

* expose `CoberturaDiff` under the pycobertura namespace
* pycobertura diff no longer reports unchanged classes

* fix incorrect "TOTAL" row counts of the diff command when classes were added
or removed from the second report.