Merge `next` branch (v0.10)

Let's give a good test to these changes and review the commits. It looks we have enough for merge.

@gmarik @starcraftman @lucc :)

## Items included

 - **[Optimisation]** Deferred rtp manipulation a.k.a. `vundle#begin`, `vundle#end`
 - **[Bug]**: Leave default rtp directories at the extremes of the final rtp
 - **[Bug]**: Make command output readable in logs (each line logged separately)
 - **[Feature]**: Support plugin pinning (don't run git commands on plugin, only rtp)
 - **[Docs]**: Add header comments to all functions
 - **[Docs]**: The big documentation update of April 2014 (includes change from `vundle` to `Vundle.vim`)
 - **[Bug]**: Detect changed remotes
 - **[Feature]**: Detect plugin name collisions (warn the user)
 - **[Docs]**: Image update in README file

## TODO

 - Before the merge, we must update the links to the documentation in the README file to reflect the correct lines of the vundle.txt doc file.
 - After the merge, tag `v0.10`