Simplify ReUnit implementation based on feedback from @travisbrown
Converted unit tests from FunSuite to WordSpec

Conflicts:
	src/test/scala/scodec/CodecTest.scala
	src/test/scala/scodec/codecs/HListCodecTest.scala
Further simplification of dropUnits thanks to @travisbrown
Added @djspiewak's lookahead and recover combinators
Added `polyxmap` and `polyxmap1` combinators based on sample provided by @djspiewak

Conflicts:
	src/test/scala/scodec/codecs/HListCodecTest.scala