Integrate BindingRedirects into Paket install process.

This PR integrates the code for #245 into the main install process. There are a few fixes I made at the same time.

Issues: -
- Does not support removal of BRs when a dependency is removed.
- Is greedy i.e. will simply put a BR for every strong-named assembly even if not necessarily required.

Could use some feedback on this as to where it should go from here (and whether I've integrated it in the correct place).