Implemented a very basic plugin system.

The way this works is pretty much the same way as `git` commands work.

When invoking `bundler <task>`, if the task isn't defined on Bundler::CLI,
Bundler will try and figure out if there is a `bundler-<task>` binary
present on the path. If so, it's invoked.

Same error message (and error codes) apply when a binary isn't found.

Also, this is just a first, simple pass at the problem. @indirect and I
had a chat about how it should work, and we should probably think about
offering the possibility of loading up Ruby plugins in the already
loaded Bundler environment.
prevent all new gems to be rubygems 2.0 only
rearrange specs
Search remote gem servers in reverse.

This reverts to the documented behavior
(http://bundler.io/v1.6/man/gemfile.5.html#SOURCE-PRIORITY) that
regressed in v1.5.0.
try to de-dupe rubygems remotes
Restore original Rubygems sources after fetching.
Detect ambiguous gems.
Encapsulate Definition#sources.
Extract SourceList class from Definition.
raise errors from option validator
Extract source URL normalization to a new method.
Separate Rubygems sources into multiple objects.

with @TimMoore and @ckrailo
lockfile source parity

the lockfile parser was creating and then providing a single rubygems
source, with multiple remotes. that doesn't work now that we have
multiple remotes, so I updated it to create source objects for all
of the remotes that are in the lockfile.

this doesn't solve the problem of source affinity at all, and at best
just punts on it, but we definitely need the lockfile parser to return
multiple rubygems sources instead of just one.
allow rubygem sources via block or :source option
Add tests for indirect dependencies.
Don't warn on system gems.
Add a spec for gems not in the pinned source.
Update Gemfile man page with new source options.
Add 1-7-stable branch to .travis.yml.
Rescue RegexpError when an invalid regexp is given to bundle show