"pycobertura show" includes highlighted source code

This pull request (PR) renders the source code highlighted according to the hit/miss lines, but only for the `pycobertura show` command. The change for `pycobertura diff` will come in a subsequent PR.

![screen shot 2014-12-28 at 12 25 03 pm](https://cloud.githubusercontent.com/assets/247661/5563725/122b38ec-8e8d-11e4-995f-adeef24687d1.png)

File paths to source code listed in the Cobertura report may or may not be present (e.g., Cobertura report was moved from where it was generated). If source files are present, they may or may not reflect the state of source when the Cobertura report was generated. Also, the `pycobertura diff` command will only have access to one version of the source, but 2 sources must be given in order to diff.

If we want to accurately show the source code for in the HTML version of `pycobertura show` and `pycobertura diff`, then we could let the user provide a base path to help pycobertura find and render the source code. The user is responsible for pointing pycobertura to the right version of the source code. CI tools and scripts that do that easily by retrieving the artifacts for the source that matches the artifact of the Cobertura report.

This illustrates a user passing the source dir:
```bash
# show
$ pycobertura show -f html coverage.xml  # assume path to file in Cobertura report is correct
$ pycobertura show -f html --source=<path-to-source> coverage.xml

# diff
$ pycobertura diff -f html --source1=<path-to-source1> --source2=<path-to-source2> coverage1.xml coverage2.xml
```