Update libguardtime to 0.3.13.

2014-03-04 release (0.3.13)
* Mostly added convenience functions for wrapper APIs:
  - functions to manage the list of CA roots from client code;
  - functions to convert hash values to and from imprints;
  - a function to extract the data hash from a timestamp.

2014-01-20 release (0.3.12)
* Publications file verification switched away from hardcoded CA root
  certificate:
  - on most platforms OpenSSL trust store is used;
  - on Windows Crypto API and its trust store are used by default,
    with option to switch to OpenSSL and its trust store.
* Now builds with both OpenSSL 0.9.8 and OpenSSL 1.0.
* Lifted a restriction of the underlying WinINet library that limited
  the number of simultaneous HTTP requests to 2 on Windows.
* Various other improvements to the build scripts.
* Replaced test timestamps with more recent ones.


(fhajny)