Added basic coverage reporting.

*Please note*: Since a coverage report build needs all kinds of
special extra files, you will need to `make clean` followed by
re-configuring with the `--enable-coverage` argument. Then
running `make coverage-report` will build you a nice report of
test coverage in the `coverage/` directory.

Note also that you will need `lcov`, `gcov` and `genhtml`
installed. On Ubuntu it's enough to `sudo apt-get install lcov`.
Added docs about testing and test coverage reporting.
Merge pull request #18 from MapQuest/coverage

Basic test coverage tool