Prevent making useless symbols.

Groups are concatenated with ':' in Bundler::Settings#without=,
and splited by ':' in Bundler::Settings#without. So replace ' '
with ':' to prevent making useless symbols and other objects.
Add :gist option for gems embedded in gists
Allow optional gem push via gem_push environment variable.
rework builds to reduce false build failures

  - stop testing 1.9.2 against 1.5.3, 1.6.2, 1.7.2
  - mark the build as passing even if 1.9.2 builds fail

MRI 1.9.2 is really. fucking. slow.

It's so slow, in fact, that running the entire Bundler test suite frequently 
takes more than 25 minutes, at which point Travis times out the build. The 
test suite itself is not actually that slow, running in 8 minutes on 1.8.7 
and 7 minutes on 2.0.0-preview2. To work around this, we no longer build the 
older versions of Rubygems against 1.9.2, and we allow failures on 1.9.2 
without marking the overall build as failed.
build against ruby-head, but allow failures

One of the big lessons of the Rubygems 2.0 release is that there  
were 90 specs that failed only when run against ruby-head and 
rubygems-master at the same time. Ruby-head with released Rubygems
and released ruby with rubygems-master both passed. So we're 
going to start building against ruby-head. That said, ruby-head 
can be kinda volatile, so we're not going to make the Bundler 
build as failing, even if the ruby-head build fails.
run rubygems hooks for and remove built :path gems
add UI#trace for backtraces without DEBUG info
whoops, build 1.8.7 against all the rubygems
try to force rake 10.0.2 everywhere
ensure the rake version by hand, because scumbag rvm
output dependent specs are such a drag
add convenience method to query if quiet option set
quiet git ouput during cloning if quiet option set
Missing word
outdated sets non zero exit status if outdated gems found #2021
make sure non-rubygems.org source keep working (@jeremy)
Make PathError message more accurate
clean up load_gemspec a bit, organize specs
add 2.0.0-preview2 to the build for now for clarity

we should replace preview2 with the latest 2.0 release as appropriate
Use different error message if file exists