[#428] additional failing tests for issue

should indicate when a duplicate project exists
[#423] tests to see if cordova add has been called in the case that platform has not yet been added to the project
[#423][#421] test for remote fallback when no platforms have been added
[#428] checks for errors before munging config.xml
[#428][#430] handle case of invalid config.xml path
[#430] tests to handle case of config.xml not found and project path already exists
[#430][#423] phonegap install now calls cordova add in event of missing platform

this does not affect the behaviour of $ phonegap local install <platform>
[#430] additional tests to detect add platform behaviour