common: trace: add AbstractIntegerEventValue
Accept providers without onInit and onFini
AbstractEventValue: add as*() and is*() methods
common: trace: add get() to Array/Dict
AbstractEventValue.hpp: add isUnknown()
Add contrib system to ease development
Merge remote-tracking branch 'simark/oninit_onfini'

Closes #4
Fix .gitmodules
contrib: run "git submodule init" from root dir
contrib: build.sh: use Bash features
ProgressPublisher: no reference as attributes
.travis.yml: try to make it work this time
.travis.yml: libzeromq3-dev -> libzmq3-dev
Build scripts: use pushd/popd everywhere
builder: do not bind if no address is provided
builder: handle all existing exceptions
builder: use builder's return value
common: trace: remove public virtual methods
TraceSet: iterator -> Iterator (typedef)
common: rpc: simplify RPC message interface