Implement parser for proposed Targets DSL.

Fixes #97
Add more Targets parsing tests
Maintain backward compatibility with standard target files by peeking ahead to the next value after reading a method + URL; if it's another HTTP method then short-circuit the header/body parsing and remember the peeked value for the next go-round
Use MatchString vs FindString
Merge pull request #98 from cwinters/targets-dsl

Maintain backward compatibility with existing target files
Use regexp.MustCompile
Update README
CHANGELOG
Remove whitespace