[WIP] Add custom code distribution

[tisba]

WARNING: This is work in progress, expect rebasing and force-pushes! Feedback is very welcome!

This pull request adds the ability to distribute compiled or uncompiled erlang modules to all nodes. The goal is to significantly simplify the usage of custom code/modules especially in a distributed setup to be used in sessions e.g. via <setdynvars sourcetype="erlang" callback="my_module:my_function">.

The proposed option to specify modules is the following:

<options>
  <option name="module_distribution" module_type="beam" value="my_module" />
  <option name="module_distribution" module_type="source" value="/tmp/my_modules/my_utils.erl" />
</options>

Module type beam will be loaded, so they have to be on the controllers load path (see command line option -X added in #92). Modules of type source are compiled via compile:file(Path, [binary, compressed, return]). Compilation errors and warnings will be logged, where compilation errors will result in tsung termination.

Any module is distributed to all nodes, including remote beams, via rpc:multicall(Nodes, code, load_binary, [Module, Filename, Binary], ?RPC_TIMEOUT) so that they are usable right away.

[tisba]

@nniclausse thanks for the hint. I think I'm also missing several spots, where new beams are launched.