Fix clang warning of potentially leaked object

Rename `dataContextCopyOrNew` to `newDataContextOrCopy` to match `new` method family.

Reference: http://clang.llvm.org/docs/AutomaticReferenceCounting.html#arc-method-families
Fix memory leak
Fix clang code-style warnings

"Method accepting NSError** should have a non-void return value to indicate whether or not an error occurred"
Fix memory leak in - [HBDataContext copy]

This class should conform to the NSCopying protocol, rather than declaring it's own -copy method.
Merge branch 'fix_clang_warnings' of github.com:randomsequence/handlebars-objc into randomsequence-fix_clang_warnings