add start of embedding crate: build system with CEF types
add MPL header
allow warnings in embedding crate, prevent constant rebuild of embedding crate

the embedding crate contains 1800+ lines of types from an external library, most
of which are not used or "rust"ically named. the former will change eventually,
but we have no control over the naming.
fix/simplify embedding build and libservo build
make base all: Makefile rule conditional, only build embedding on non-android
auto merge of #2257 : zmike/servo/embedding-base, r=larsbergstrom

Another attempt at the merginating of my long-dormant embedding work for #441

This adds the basics for the embedding crate: build rules and 1800+ lines of CEF types

r? @larsbergstrom