Update a figure in use case summary

[k-toumura]

For issue #195.
I created an updated version of 'Figure 14 Use Case Overview' (not overwrite .png, but created new file).
@mlagally @mkovatsc Please review and comment if there are any problems.

[k-toumura]

• Update 4.2 Common Patterns and 4.3 Summary to avoid enforcing server paradigm on devices.
  • In 4.2, use client/server role as roles in transport protocol layer, and show a device acts as both of roles.
  • Section 4.3 describes that application layer treat a device as provider of an abstract interface for interaction, regardless of roles in transport protocol layer. The requirements in Section 5.2.2 Devices can be derived from this.
• Minor editorial fix (closing tag, duplicated id)
• update link of Figure 14.

[mlagally]

@k-toumura Many thanks for your updates, this looks good.
There is a minor respec issue (a reference to WoT Servient), but this will go away,
when my PR is merged.

Perhaps an additional sentence could state that "the use cases and WoT architecture does not enforce specific client or server roles on system components".