Update for iron 0.6 (#141)

* Adapt error handling to changes in iron
* Fix compilation warning about unnecessary `mut`
* Bump crate version for release

Version 0.5.1

Version 0.5.0

(release) 0.4.0

(release) 0.3.4

(release) 0.3.3

(release) 0.3.2

Version 0.3.1

(release) 0.3.0

(release) 0.2.0