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
Updated default DOCKER_PORT in nat rule

the `DOCKER_PORT` can be changed for the `host` but have to be `4243` in the guest
add Option to get settings from external config file
if you kill the VBoxHeadless process, the state is set to aborted (happend to me when I ran out of memory) - with this PR you can delete it as though it were stopped on purpose
Merge pull request #98 from SvenDowideit/allow-delete-of-abort-status-vm

if you kill the VBoxHeadless process, the state is set to aborted (happe...
check if vm exists
Merge pull request #97 from aheissenberger/feature/pull/optional-config-files

get settings from external config file if exists
Merge pull request #94 from moul/patch-4

Updated default DOCKER_PORT in nat rule
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