Make 'rebar xref' honour the skip_app directive

It is now possible to call rebar as:

  rebar xref skip_app=Mod1,Mod2,...

This makes it easy to skip running xref on (e.g)
imported dependencies in your application.

The function rebar_utils:is_skipped_app/0 is added so
that other rebar commands may use it.
Add Torbjorn Tornkvist to THANKS file
rebar_xref: move code to proper place
Add rebar_utils:deprecated/4 and remove define
Move is_skipped_app/0 to rebar_app_utils
Universally support apps=/skip_apps=
Fix help text formatting
Fix rebar_core crash (reported-by: Jeremy Raymond)
Remove trailing whitespace in rebar_reltool
Trim reltool config and add nodeid to release
Comment out vm args
bootstrap: generate windows scripts
bootstrap: do not ignore write_file return value
Add support for customising common test directory

This patch allows users to specify the directory in which common_test
source files can be found. Most common_test suites are integration,
rather than unit tests and keeping the sources apart from test sources
for other frameworks such as eunit and PropEr is a useful feature.
Move vcs_vsn/2 to rebar_utils
Add {vsn, _} support for reltool.config
Add Ali Sabil to THANKS file
Don't kill any inet_gethost_native after eunit

Avoid error reports like this one:

  prompt$ rebar eunit
  ==> dummy (eunit)
    Test passed.

  =ERROR REPORT==== 29-Dec-2011::23:22:11 ===
  ** Generic server inet_gethost_native_sup terminating
  ** Last message in was {'EXIT',<0.62.0>,killed}
  ** When Server state == {state,inet_gethost_native,undefined,<0.62.0>,
                                 {local,inet_gethost_native_sup}}
  ** Reason for termination ==
  ** killed

Previously, it could happen if an eunit test did something that
required a name lookup, like in this example:

  -module(dummy_tests).
  -include_lib("eunit/include/eunit.hrl").

  x_test() ->
      {ok, _Hostent} = inet:gethostbyname(localhost).

The inet_gethost_native is a process, started on demand, under a
supervisor_bridge under the kernel_sup, but it is not a gen_server
process or the like, so it has nothing in '$ancestors' in its process
dictionary to indicate it is part of kernel.
Merge pull request #168 from tomas-abrahamsson/eunit-inet_gethost_native

Error report killing inet_gethost_native after eunit
Add Tomas Abrahamsson to THANKS file