Improvements to rtp management.

Hi, this is long overdue we discussed it during the Plugin change. This combines several changes listed below to rtp flow. Thanks to @lucc for his rtp reordering snippet, works well.

Several changes:
 * For both Bundle and Plugin fixes the rtp ordering issues by calling rtp_add_defaults, the default rtp is preserved before/after vundle changes.
 * For Plugin, changes command to only cache the name/opts in g:bundles.
 * On calling PluginInit, performs one time initialization of
 all bundles in g:bundles then modifies rtp.
 * Eliminated useless config#init function.


* To fix Bundle command rtp, I call rtp_add_defaults() every time, doesn't seem to add noticeable overhead.

TODO:
* Want to do more testing to make sure everything remains functional.
* I'd like to know if the plugin changes make any noticeable speedup, I've seen about 30-40ms I believe on my slowest machine. I'd like to know if it's consistent on other machines.

@gmarik @jdevera probably want to look at this.