`shared_library` support

gyp.js currently supports static_library, executable, and none target_type, but shared_library is very important for Node.js addons! Therefore we need to support it in ninja.

Hints:

• New ninja rule solink needs to be added here
• This rule should be used there

Possible problems - many.

Support:

• OS X
• Windows
• Linux
• FreeBSD (needs verification, should work)
• Others (needs verification, should work)