Fix deprecation warnings
Remove support for Ruby 1.9
Avoid creating Symbols based on user input

This removes a potential security issue: if untrusted users are allowed
to have templates compiled, they could use random reference names to
allocate memory that won't be reclaimed by the garbage collector.

This changes the API of the `method_available?` protocol a bit, passing
a String rather than a Symbol.
Merge pull request #70 from zendesk/dasch/dont-cast-to-symbol

Avoid creating Symbols based on user input
Describe parameterized variables in the README
Add "parameterized variables" to the ToC
Describe `setup!` in the README
Add the memleak fix to the changelog
Refactor Compiler
Add stricter parameter requirements
Extract a ReferenceCompiler
Allow an optional parameter
Fail properly when a method has an invalid signature
Initialize with the method name
Move more compilation logic into ReferenceCompiler
Allow setting attributes on references
Add a changelog entry for reference attributes
Require Ruby version 2.1

Version 2.0 does not support required keyword arguments, which is needed
in order to support required attributes.
Merge pull request #69 from zendesk/dasch/new-kwargs

Support for attributes on references using Ruby 2's kwargs
Describe the new variable attributes