[fix] Build system. Fixes #181

This fixes build calls from outside the build folder. Now we have:

## Build: Create zip extension package

### From extension root
```bash
phing -f build/build.xml
```

### From build folder
```bash
cd build
phing
```

### From build folder using bash script
```bash
cd build
./build.sh
```

## Build: update test site

### From extension root
You have to duplicate `build/build.properties.dist` to the extension root `build.properties` and then adjust the content to your test site path like:

```ini
# Properties files are configuration files used by PHING scripts containing:
# key=value

# Testing Joomla site localhost folder used by build.xml
joomla.dir=/var/www/jcms3x
``` 
Then you can update the test site anytime with:
```bash
phing -f build/copier.xml
```

### From build folder
You have to duplicate `build/build.properties.dist` to `build/build.properties` and then adjust the content to your test site path like:

```ini
# Properties files are configuration files used by PHING scripts containing:
# key=value

# Testing Joomla site localhost folder used by build.xml
joomla.dir=/var/www/jcms3x
``` 
Then you can update the test site anytime with:

```bash
cd build
phing -f copier.xml
```