ctx_correct.c: add option --print-orig

1. ctx_correct.c: added -P,--print-orig option to add original sequence to
   read name:

   >read1 orig=CAGATGAGTT
   CAGATGACTT

2. graph_step.h: Add graph walker statuses:
 * GRPHWLK_POPFWD         --  Success: fall back to pop, only one choice
 * GRPHWLK_POPFRK_COLFWD  --  Success: fork in pop, only one choice in colour

3. PhiX/about.txt: add readme on data source for empirical read error distribution

4. split out assembly statistics to assemble_stats.{c,h}

5. assemble_contigs.c: Fix graph walker status counting in contig assembly:
   path decisions were being recorded before we have decided to keep contig.

6. assemble_contigs.c: record number of contigs halted by RepeatWalker