add bundle_bin option

[Arcath]

I had an issue with mina where it wasn't picking up on the path for my gems so my deployments where failing on the bundle command.

This change just adds a bundle_bin option to allow you too set a custom path to the bundle binary. In my deploy.rb I use:

set :bundle_bin, '/var/lib/gems/1.8/bin/bundle'

which solves my problem.

Other than this mina working great :D thanks for writing this awesome tool!

[Arcath]

Mina wasn't seeing my bundler command so kept failing to deploy.

Not a massive change which to 99% of people wont make difference but its just useful to be able to alter the path if for what ever reason your path changes dont apply or you want to use a specific bundler installation

[rstacruz]

Thanks!

Another option is to add export PATH=/var/lib/gems/1.8/bin:$PATH to your .bashrc, since .bashrc is now read @ v0.1.2.pre1.

[Arcath]

I do have that already in my .zshrc but it didn't seem to help.

[rstacruz]

Mina always uses bash.

Also, the bashrc feature isn't available on the main release yet (it's a pre-release) :)