Make install-prereqs-ubuntu.sh work with newer versions of Ubuntu

Currently, the following setup pages both have something missing when trying to compile or simulate.
http://dev.ardupilot.com/wiki/building-px4-for-linux-with-make/
http://dev.ardupilot.com/wiki/setting-up-sitl-on-linux/

I tried to fix this by making the *install-prereqs-ubuntu.sh* correctly install everything that's needed. Here are the changes I did:
* Install libraries for 64bit Ubuntu systems.
* Less noise apt-get output
* Install packages required for SITL simulation
* Install pymavlink, MAVProxy and droneapi
* Remove modemmanager package
* Add ```Tools/autotest/``` to the path
* Support for relative paths
* Install ARM toolchain at ```/opt```

The script has been tested in a VM running a clean install of *Ubuntu 14.04 LTS*. 

### How to use it
The steps required for installing the environment are: 
(please not that you will need to checkout my *ubuntu_script* branch while this PR is not merged)
```
# Install git
sudo apt-get -qq -y install git

# Clone the source
git clone https://github.com/diydrones/ardupilot.git

# Run the install-prereqs-ubuntu.sh script
ardupilot/Tools/scripts/install-prereqs-ubuntu.sh -y

# Reload the path (log-out and log-in to make permanent)
. ~/.profile
```

To build the code (e.g. AC for pixhawk):
```
# Build for ArduCopter
cd ~/ardupilot/ArduCopter
make configure
make px4-v2
```

To run a SITL simulation:
```
cd ~/ardupilot/ArduCopter
make configure
sim_vehicle.sh
```