Various improvements

With these changes merged, we can do fun stuff [like this](https://github.com/staticfloat/BinaryAnalysis.jl/blob/master/test/runtests.jl) (Where all the actual work happens [here](https://github.com/staticfloat/BinaryAnalysis.jl/blob/master/src/BinaryAnalysis.jl)).

I assume you'd eventually like to make ObjFileBase flexible enough that I can abstract out the mach-specific stuff and do this to ELF files as well?
I was also a little unsure about this, until I noticed the mixing of `ccmd` and `cmd.cmd` in `readloadcmd()` which was causing a lot of confusion, and some loadcommands to sneak through unrecognized.

Since the purpose of `LC_REQ_DYLD` doesn't really have any import on our reading of the files, I think it's okay to disregard it in this way.