Allow `uru .` to use .ruby-version

[Luis Lavena]

chruby, rvm and rbenv have normalized in the .ruby-version schema:

https://gist.github.com/fnichol/1912050

Since version activation is on-demand, I thought the usage of uru . could trigger the parsing of .ruby-version file found in current directory.

In rbenv for example, it will lookup for .ruby-version in current or parent directories until it reaches $HOME

This will allow easy switching to a particular version when moving between projects.

Thank you.

[Jon]

I also like what I've read on .ruby-version and think uru . is a very clever idea.

I originally was thinking about if/how to hook cd, but this looks much cleaner

[Jon]

• write private walkUp helper similar to go's filepath.Walk()
• walk up from . to root (no cross volume skipping for windows) and use contents from first .ruby-version found; look in $HOME | %UserProfile% as the final location check.
• exact match (no fuzzy matching) on ruby registry ID and Exe fields to support the following (single line) .ruby-version styles:

2.0.0-p457
ruby-1.9.3-p430
jruby-1.7.4

• since ID and Exe don't uniquely identify a ruby, ensure support for multiple exact matches, i.e ruby-2.0.0-p197 could match both x86 and x64 registered rubies.
• get real-world .ruby-version samples from chruby/rbenv/rvm users to see if previous support needs changed. update: appears chruby and others use fuzzy mapping.
• ensure a corresponding GEM_HOME is generated for the switch
• TODO: how to handle jruby and ruby 1.8.7 version wackiness; their ID values look like 1.7.4 and 1.8.7 and don't contain patch info because the current regex parsing doesn't grok their funky version strings. Revisit current regex

[Jon]

impl started on rebase friendly use-dot branch

[Jon]

Initial .ruby-version support. Closes #23

Support exact matching against ID and fuzzy matching against TagLabel when .ruby-version contains single line contents such as:

200 1.7.4 2.0.0-p197 200p197-x64 system

→ <<cset bbc050626fcc>>