Refactor following bundle command
Add bundler parallel install option
Add parallel gem for detecting the count of cpu cores

Use the count of cpu cores for `bundle install` parallel jobs,
because bundler v1.4.0.pre.1 provides parallel install option.
Make Travis build against more branches and rubies
Allow failures on JRuby and ruby-head for now
Complain if no Appraisals file is found.
Add missing integration test on missing Appraisals

Also, rename exceptions.rb to errors.rb
Refactor and private `Appraisal#install_command`

This command should not be called directly outside Appraisal, so it does
not make sense to leave it as a public method.
Shift `ARGV` when re-running the command

In some circumstances, `ARGV.slice` may return nil. It's safer to just
do `ARGV.shift` and discard the first object in the Array, since in
the end the resulting object would still be an array.

Fixes #54, Fixes #59.
Update gemspec styling
Add my name to gem author
Add thor and bin/appraisal executable
Setup RSpec to support acceptance testing
Add base spec for `appraisal generate`
`appraisal generate` is now working
Add default task, so `bin/appraisal` => `generate`
Speed up some test by build gem only once
Fix cucumber test failure
Adds `appraisal install`
Do less RSpec overriding