xmerl needs to be started in examples

[thraxil]

On ubuntu precise, with R15B01, attempting to run the examples gives:

./examples/cowboy_echo.erl
escript: exception error: no match of right hand side value {error,{not_started,xmerl}}
  in function  cowboy_echo:main/1 (./examples/cowboy_echo.erl, line 14)
  in call from escript:run/2 (escript.erl, line 727)
  in call from escript:start/1 (escript.erl, line 277)
  in call from init:start_it/1 
  in call from init:start_em/1 

I think cowboy requires xmerl to be started first.

[majek]

Thanks for the patch.