Checking return of VBoxManage modifyvm

Checking return of `VBoxManage modifyvm`

Some options may be unavailable in prior version of `VirtualBox` causing `modifyvm` to fail

Without this patch, the script continue with an unconfigured virtual machine without enough ram to boot the kernel
check if vm exists
add VM_MEM to the customisables, and document the ./boot2docker/profile file in the README
Merge pull request #100 from SvenDowideit/b2d-profile

add VM_MEM to the customisables, and document the ./boot2docker/profile ...
Merge pull request #99 from aheissenberger/feature/pull/check-if-vm-exisits

check if vm exists
Merge pull request #90 from moul/patch-2

Checking return of VBoxManage modifyvm
update OSX docker client install
OSX: install docker client with Homebrew
add a heading
Merge pull request #102 from SvenDowideit/pr/76

README / update OSX docker client install
Merge remote-tracking branch 'upstream/master'

Conflicts:
	boot2docker