Fixed string length decoding

Fixes #25
Merge pull request #26 from vhbit/str-len-decoding

Fixed string length decoding for up to 255-bytes sized strings.
Modify .travis.yml to use new rustup URL.

The current .travis.yml tries to use the one on www.rust-lang.org,
instead of the one on static.rust-lang.org. This fixes that.
Merge pull request #27 from TimDumol/travis-static-rustup

Modify .travis.yml to use new rustup URL.
Updated to Rust nightly
Merge pull request #29 from thehydroimpulse/rust-master

Updated to Rust nightly
Replace hard-coded numerical limits by MIN/MAX constants
MsgPack shouldn’t expect buffer is owned

This way it allows to use both owned and borrowed data and is much more flexible.
Merge pull request #31 from vhbit/mem-agnostic

Be agnostic on is it borrowed or owned data