Add a CONTRIBUTING file

Allow would-be contributors to read up on the project guidelines.
Fix the Markdown in CONTRIBUTING
Rubinius 1.9 is no longer supported by Travis
Run tests with Ruby 2.1
Specify what happens when a required param is not specified
Merge pull request #54 from zendesk/dasch/test-coverage

Specify what happens when a required param is not specified
Add conditional blocks to templates
Conditional blocks and inverse conditional blocks are add,
along with their proper tests.
Merge pull request #53 from redjazz96/conditional-blocks

Add conditional blocks to templates
Extract each exception and add a common base class

It's now possible to rescue Curly::Error and get all the compilation
errors.
Remove dead variable
Merge pull request #55 from zendesk/dasch/cleanup-errors

Extract each exception and add a common base class
Refactor Curly::Compiler
Update README with conditional blocks
Expand the section on escaping Curly references
Merge pull request #56 from zendesk/dasch/conditional-blocks-documentation

Add documentation for conditional blocks
Make sure CompilationError and SyntaxError inherit from Error

This allows users to rescues all Curly exceptions by rescuing
Curly::Error.
Release 1.0.0rc1
Fix confusion in the README
Remove unused exception class
Document the exceptions raised by Curly::Compiler