Bump version to 0.2.3

Changes since 0.2.2:

* Nikolay Bachiyski: Record extra information as environment variables (7c1f44f)
* Nikolay Bachiyski: Extract parsing the environment into a function (2bc73eb)
* Daryl Koopersmith: Rename prompt to shell in CLI. See #60. (fdd8cfc)
* Nikolay Bachiyski: Rename impromptu.options.prompt to shell (133ca23)
* Daryl Koopersmith: Generate Impromptu paths in constructor. (44f8994)
* Daryl Koopersmith: Record prompt errors to a log file. (d53568f)
* Daryl Koopersmith: Handle compiler and runtime errors. (1aac905)
* Mark Jaquith: Tell OS X Terminal about the current directory (d0b85af)

In particular, the error logging is really valuable for debugging problems. Also the new environment variables are being used in the system module.