Implemented new getMultiple pattern for all instances in Block: dataArrays, dataTags, sources, simpleTags
Implemented new getMultiple pattern for the one instance in DataArray: dimensions
Implemented new getMultiple pattern for one instance in DataTag: references. To achieve this a new 'getReference(size_t index)' was added that accepts an index instead of an id and that checks whether block has the referenced data array.
Implemented new getMultiple pattern for last instance in DataTag: representations.
Comeback of findSources in Block, implemented by using Sources::findSoures. This implementations allows duplicate findings however.
Implemented new getMultiple pattern for all instances in File: blocks, sections.
Implemented new getMultiple pattern for all instances in Section: sections, properties.
Implemented new getMultiple pattern for all instances in SimpleTag: references, representations.
(Therefore in analogy to DataTag a "getReference(size_t index)" was implemented)
Fixed errors appearing in "File" test case
util: initialise the rng inside a std::call_once

This makes it threadsafe.
util: explicitly cast to unsigned int when initialising the rng

Closes issue #77
Merge pull request #79 from gicmo/win_rand

Looks all OK
Merge pull request #75 from balint42/master

Implemented new getMultiple pattern for all instances in Block

This was quite allot: thanks. 
As far as I can tell everything is OK.
[travis] Use ctest for building the project

Using ctest for everything has the advantage that the all the
individual steps (configure, build, test) will be submitted to the
dashboard, including all the warnings generated in all steps
[cdash] change project name to nix
[travis] Silence unused arguments warning for clang

Happens during linking, but we are not interested in it.
[travis] Use COMMIT instead of BRANCH for build name
[cdash] shorten commit id in build name to 7 chars

And remove "travis" from build name (that is clear from the site).
[cdash] attach travis configuration to the submission
[travis] enable irc notifications