Implemented package descriptions for cargo

[startling]

This solves issue 1601.

Descriptions are taken from a description field for each package in the packages.json file of a cargo repository. I also submitted a pull request to cargo-central with these fields; for now, though, you can point your sources.json to https://raw.github.com/startling/cargo-central/descriptions/packages.json and see how it works.

Example output:

$ cargo list
info: central/rustray (c89c5834-63db-4a6e-8e0a-d75985b1730e) [example]
   >> A raytracing proof-of-concept in Rust.

info: central/rust-hello (83941200-18b3-41b1-8316-b98a2950b33a) [example]
   >> Hello World in rust.

info: central/crypto (38297409-b4c2-4499-8131-a99a7e44dad3) [crypto]
   >> OpenSSL libcrypto bindings for the Rust language.

info: central/zlib (6c34ea67-f290-4438-9d82-db480114d7da) [compression]
   >> zlib bindings for rust.

info: central/pcre (54aba0f8-a7b1-4beb-92f1-4cf625264841) [regex]
   >> PCRE bindings for Rust.

info: central/tnetstring (ce93b70c-c22a-45fa-97a7-66ab97009005) [serialization]
   >> tnetstring serialization library for rust.

info: central/zmq (54cc1bc9-02b8-447c-a227-75ebc923bc29)
   >> Rust zeromq bindings.

info: central/sqlite (5dd017c1-a8fb-458f-b430-cecf793ffbd5) [database]
   >> SQLite3 Bindings for Rust.

info: central/rustx (ec9ace30-b7dc-4d66-9cce-a21cc1c06c93) [utility, script, shortcut]
   >> Scripts to compile and run Rust programs in one command.

info: central/csv (c88f4e89-fc12-4cb3-a978-35d135aefcfd) [spreadsheet]
   >> CSV implementation for Rust.

info: central/iter (1ADE62EE-BE76-46C1-B065-3438B7199A37)
   >> An iteration library for Rust.

[brson]

Thanks!