Fixes #33: Provide custom delimiters to the compiler
A few tweaks to default delims patch (and some tidying):

- triple-stache mode reverted to only work with {{{ }}} rather than working
  with whatever default delims you set; I think this is more in the spirit of
  the Mustache spec

- Compiler.withDelims takes a delims string rather than the internal Delims
  class which callers don't care about or have access to

- nixed Delims constructors, made updateDelims fluent

- nixed DelimsTest, it wasn't testing anything that wasn't also already tested
  by the functional tests in MustacheTest