added NAs to fvecs. Currently NAs encoded as Fixed value, 255 for bytes, Short.MIN_VALUE for 2 bytes values, Integer.MIN for ints, and Float.NaN and Double.NaN for floats/doubles.

Also changed the compression in NewChunk.compress().
Merge branch 'master' of github.com:0xdata/h2o

Conflicts:
	src/main/java/water/fvec/C4Chunk.java
	src/main/java/water/fvec/C4FChunk.java
	src/main/java/water/fvec/C8Chunk.java
	src/main/java/water/fvec/C8DChunk.java
	src/main/java/water/fvec/Vec.java
added back my previous changes for NAs (lost them during merge).