Support compiling to WASM

[seanaye]

Currently httpmock always compiles with the server enabled (there is no feature to enable/disable it)

This prevents the crate from running with wasm-bindgen-test because the server uses a bunch of things that cannot compile in wasm32 environments. (e.g. we can't bind network sockets in wasm)

It would be nice if we could use the connect method in wasm to connect to an a standalone server