modified renderable to have the option of falling back to text if the partial is missing. this is in line with the docs that read 'If a view file does not exist, then p_a falls back to the old behaviour and tries to translate the activity :key using I18n#translate method (see the section below)'. However - instead of doing it by default it only exibits this behaviour when :fallback => :text is provided. This way the other tests that rely on ActionView::MissingTemplate be raised still pass. README updated to match new implementation.
modified the :fallback parameter to allow a default view to be used instead of raising an exception. README updated.
updated YARD docs providing information on how to use the :fallback parameter. Closed some dangling open brackets in the README.
Merge pull request #148 from shorrockin/master

customizable behaviour for activity rendering in the view