Read Skeleton from skel file

Read Skeleton (or SoftSkeleton) from skel file. Added following two functions:
```c++
static dynamics::Skeleton* SkelParser::readSkeleton(const std::string& _filename)
static dynamics::SoftSkeleton* SoftSkelParser::readSoftSkeleton(const std::string& _filename);
```

The skel file should start with `<skeleton>` tag instead of `<world>` tag. For example,
```xml
<?xml version="1.0" ?>
<skel version="1.0">
    <skeleton name="box skeleton">
        <body name="box">
            <gravity>1</gravity>
            <transformation>0 -0.325 0 0 0 0</transformation>
            <inertia>
                <mass>1</mass>
                <offset>0 0 0</offset>
            </inertia>
            <visualization_shape>
                <transformation>0 0 0 0 0 0</transformation>
                <geometry>
                    <box>
                        <size>0.1 0.05 0.1</size>
                    </box>
                </geometry>
                <color>0.8 0.3 0.3</color>
            </visualization_shape>
            <collision_shape>
                <transformation>0 0 0 0 0 0</transformation>
                <geometry>
                    <box>
                        <size>0.1 0.05 0.1</size>
                    </box>
                </geometry>
            </collision_shape>                                
        </body>
            
        <joint type="free" name="joint 1">
            <parent>world</parent>
            <child>box</child>
        </joint>
    </skeleton>	
</skel>
```