Changed `git` and `dev` to not check dependencies when installing.

They are still checked in `path`.

Rationale:

 * Many git repos or development directories do not have a valid haxelib.json file
 * Even if they do have a valid haxelib.json file, the dependencies may point to a development version of the dependency - not one that is available on lib.haxe.org.
 * Dependencies are still evaluated at during "haxelib path", so using "-lib mydevlibrary" will still evaluate dependencies correctly.
 * If a dependant library is not available, trying to compile will give "Library mylib is not installed : run haxelib install mylib".  This is pretty acceptable IMO.

Also:

 * More unit tests for Data.hx
 * Better error handline in Data.hx
 * I had to get rid of some of the error-handling/fallback stuff I'd started to implement in Data.hx now that I adopted this other strategy.

Affected Issues:

Fixes #40
Fixes #48
Fixes #50 
Fixes #51 (Needs testing)

and the tests help make progress on Issue #38