Full libxml2 coverage via bindgen

[0.2.1] 2018-23-07

Added

• Node::set_node_rc_guard which allows customizing the reference-count mutability threshold for Nodes.
• serialization tests for Document
• (crate internal) full set of libxml2 bindings as produced via bindgen (see #39)
• (crate internal) using libxml2's type language in the wrapper Rust modules
• (crate internal) setup bindings for reuse in higher-level crates, such as libxslt

Changed

• NodeType::from_c_int renamed to NodeType::from_int, now accepting a u32 argument

Removed

• Removed dependence on custom C code; also removed gcc from build dependencies