add default gem bindir to PATH

[Jon]

No description provided.

[Jon]

Initial implementation with the following behaviors:

• pre-calculate sane default GEM_HOME (always exported) similar to:

if runtime.GOOS = "windows" {
    GEM_HOME = %USERPROFILE%\.gem\${ruby_exe}\${ruby_version}
} else {
    GEM_HOME = $HOME/.gem/${ruby_exe}/${ruby_version}
}

• upon switch to non-system ruby, prepend base PATH with GEM_HOME, new ruby PATH, and canary similar to:

%GEM_HOME%\bin;${new_ruby_PATH};;;...
    -or-
$GEM_HOME/bin:${new_ruby_PATH}:::...

[Jon]

Initial implementation appears to be working on Windows and Ubuntu 13.04