Convert specs to RSpec 3 syntax with Transpec

Converted specs to 3 syntax using the Transpec gem. Additionally, updated test in endpoint_spec.rb on Line 108 to check for nil rather than attempting to match on nil.
Merge pull request #619 from danielspector/transpec-rspec-update

Convert specs to RSpec 3 syntax with Transpec
Adds Guard with RSpec/RuboCop
Update README.md
Merge pull request #623 from edelgado/master

Added a Table of Contents to README.md
Moved buidl status above the fold.
Adds Grape::Endpoint.before_each for pre-run endpoint config. Refs #396
Merge pull request #397 from intridea/endpoint-setup

Adds Grape::Endpoint.setup for pre-run endpoint config.
Add mutually exclusive params validation
Merge pull request #626 from oliverbarnes/mutually_exclusive

Add mutually_exclusive params validation
Add Hack on Nitrous Button
Merge pull request #630 from arunthampi/master

Add Hack on Nitrous Button
Fix: Grape::Endpoint#present causes ActiveRecord to make an extra query during entity's detection.
added exactly_one_of validation

updated readme

updated changelog

make exactly_one_of inherit from mutually_exclusive, add test

fixed typo, small changes
Merge pull request #637 from Morred/exactly_one_of

added exactly_one_of validation
Wrong date in CHANGELOG.
Added support for blocks with reusable params.
Use a specific RBX version, attempt at fixing CI.
Just build latest rbx-2.
The rescue_from keyword now properly defaults to rescuing subclasses of exceptions.