Add a "project" argument to Pytest.vim

This runs tests for the entire project. It does this by starting in the
current directory and walking up to '/' looking for a directory called 'tests'
or a file called 'tests.py'. If it finds either of these, run py.test
otherwise error out letting the user know that there are no tests defined for
the project tree. There is a helper argument 'showprojecttests' which shows
where in the directory tree we will run py.test
Re-do ProjectPath() to use finddir instead of walking the directory tree
ourselves
Add project test object to Completion function and rename the function that echos
the project test dir (where py.test will be executed) to projecttestwd
Add documentation for the project and projecttestwd targets
Update README to include project explanations
Merge pull request #14 from tabashco/add-test-project

Add a "project" argument to Pytest.vim