issues with readme: explain `type` and correct `ios7`

[fyears]

Here are two issues with README.md and README.rst.

In the beginning, I didn't know whether type parameter is used for specifying the type of input files or output files. Only after trying did I know that it is used for specifying the type of output file. Maybe you could add some explanation to it.

And in your readme in github, you write [--type {...ios7..}]. But it seems that you do not write any relevant code. And the one I installed using pip even do not support this parameter. I suggest adding some code, treating ios7 and ios8 as the alias of ios6, although I AM NOT sure whether the database format has changed or not.