[QRS detection] Read and write functions

* Changes:
  - Read and write signals from and to file implemented.
  - Added testdb with one test sample.
* Notes:
  - Test samples will be acquired from:
    http://www.physionet.org/physiobank/database/ecgiddb/
  - Downloaded samples are converted from binary format
    to standart format with wfdb-tool available on:
    http://physionet.caregroup.harvard.edu/physiotools/wfdb.shtml