Enable the net_http probe by default
Update to rust master

* Do not double copy when serializing
* Remove old Rust types
* Remove annotation header functions
* Add new unit tests around Numbers, Strings and Structs
Use uint32_t instead of int
Inline rust_support macro and prototype
Safely fail if library/headers are not found

Fixes #154
Depend on rspec < 3
Use new rust library
Release 0.3.14