A few improvements to the documentation

[mjkillough]

This PR makes a bunch of changes to the documentation in order to fix:

• Make all references to the log crate be hyperlinks #11: Make reference to log crate into a hyperlink. Add an extra line to the summary at the beginning of the module-level documentation to indicate that this crate should be used with the standard logging facade (unlike something like slog).
• Consistently use monospaced font for RUST_LOG.
• Add missing Errors and Panics section to documentation #10: Add Errors/Panics section to init()/try_init() docs.
• Do not unwrap in examples #9: Avoid unwrap() in Builder doc string.
• Add missing rustdoc examples to Logger::new and Logger::filter #8: Provide examples for Logger::new() and Logger:filter(). Expand doc string for Logger to give more context and links to the typical ways of constructing it.

Please let me know if you'd like these changes as separate PRs - they are already separated by commit.

This PR needs the changes from #15 before it will compile or pass tests. (I've merged the two branches locally and confirmed both cargo check/cargo doc and cargo test produce the correct output).

[sebasmagri]

Thanks!