Indent rule enforces indentation with only tabs or only spaces

See https://github.com/palantir/tslint/pull/212 .

Finally i found the time to fold the indent-with rule into the indent one. 

As @ashwinr is going to work on the indent rule, i preferred to leave the test files named indentwith_{spaces|tabs}*, so you can add a new one which just tests the indentation size.


Please let me know if it's ok like this or if this needs some more work.
i think it's cleaner to be explicit and always require "spaces" or "tabs" as an option instead of any default. do you have any objections to that?