correct a grammer in perl module
add an option for defining transcript priors in Gibbs sampling

- User can use the option "--prior" to sepcifiy prior information to be used in
  Gibbs sampling

- In the prior file, the first line, usually defining the header information
  will be ingored. The next M lines specify the prior for each transcript.
  Priors must be ordered in the same manner as in the .ti file
  M = the total number of transcripts