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
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
fix broken get last release version function if curl UA is set to
curl -A "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)" 'https://api.github.com/repos/steeve/boot2docker/releases'
this will return a JSON with windows line feeds
add more error handling
Merge pull request #107 from aheissenberger/fix/pull/get-latest-release-wrong-ua

boot2docker / fix broken get last release version function if curl UA is set to IE
Fix for 29a9bca5c: Commit introduced non POSIX (bash only) commands

The commit 29a9bca5c74d7305e81e0144b0252bf619a0bb6a does not work under
a POSIX shell like dash (Debian/Ubuntu). This code fixes the problem
and makes the script POSIX compliant.
Minor fix: Some plattform don't support the --vtxux for VBoxManage
Merge pull request #112 from dol/hotfix/posix-fix-for-debian

Fix for 29a9bca5c: Commit introduced non POSIX (bash only) commands and breaks on Debian (dash=/bin/sh)
Merge pull request #113 from dol/hotfix/optional-vtxux-support

Minor fix: Some hardware architecture don't support the --vtxux for VBoxManage modifyvm