raise informative error message when user supplied invalid platform

Make PodsDummy class unique to each target

When generating the PodsDummy class, make that class unique to each target by using the Target Definition's label.

Closes #398.

Unfortunately, OS X >= 10.7 ships with 1.3.6 :(

Change git clone command to use double quotes instead of single quotes

Added clean output from pod spec create as an example, to be referenced in the wiki

Closes #392.

Incidentally touched ALL THE FILES.

Closes #263

Gender-neutralise podfile.rb docs

[ci skip]