Improve the dev-manual regarding the compiler files section

Fix #721
More generic compiler URLs

Now package and compiler urls are handled in the same (eg. you can have hg/git/darcs/local/http compiler urls).

The only difference is that local directories are linked instead of rsynced to be quicker (and support compiler hacking tricks)
Update the dev manual for .comp files
If an `opam` file is located at the root of a dev package, use its build instructions instead of the one stored in OPAM global state.

Fix #671