This fixes issue google#311. The mapped address of libraries was never considered, nor was the load address of the segments.

There is already a function very similar to getTextAddress(). This also adds a test that tries reading symbols / source addresses from a library and an executable.

I also had to fix a couple of bugs for the tests to work:

1. Crash when the regex passed to f.Symbols() was nil.
2. The last symbol returned by `nm` is always discarded.

* Return an error if __TEXT isn't found
* Return an error if start - textSegment.addr would underflow
* Break out findSymbol() function
* Rework findSymbols() function, it now returns non-EOF errors instead of ignoring them
* Remove accidentally added debug binary