automatically handles text and binary has_prefix_files

I have been working on a somewhat complicated conda package (qt5) which has required several tweaks to how conda/conda-build deal with hard-coded file paths in order to relocate the library. I just realized that the accumulation of these tweaks would allow conda-build to automatically identify binary and text files with such hard-coded paths, such that it would not be necessary to list them in `meta.yaml` under `has_prefix_files` and `binary_has_prefix_files`. This pull request implements this feature, and it actually simplifies conda-build's `build` and `config` modules somewhat.

However, this feature requires using a long build prefix for all packages, since conda-build would not know whether the long build prefix were necessary until after installing. At the moment, conda-build (master) inspects `meta.yaml` to see if there are any `binary_has_prefix_files`, and if so, it uses a long build prefix padded to 80 characters, like `_build_placehold_placehold_[...]`, in order to give enough space to insert the installation path when conda install relocates the library. I submitted a pull request a while back that simply changed the build path to something long, and it was accepted but changed to use a shorter build path when the long one was not needed. This added a fair bit of complexity to conda-build's configure module, and created an issue that is still open (conda/conda-build#191).
It would be clearer to say "included in meta.yaml" or something like that. 

Let's leave it as an error for now, but if people complain we can add an option to change it to a warning.