make servo::run() and opts::Opts.n_render_threads public for use in embedding crate
fix build of embedding crate

I blame @larsbergstrom for this failure.
serious mode engaged: start of embedding crate using FFI

current status

=============

[X] Successfully crashing CEF

[ ] Successfully not crashing CEF
add readme for using embedding crate
auto merge of #2408 : zmike/servo/embedding-invasion, r=larsbergstrom

This adds further support for crashing CEF and world domination.

@larsbergstrom