Emit error codes when reporting diagnostics.

Addresses part of issue #506.

Before:
```
file.ts(1,6): ';' expected. 
file.ts(1,11): ';' expected.
```

After:
```
file.ts(1,6): Error TS1005: ';' expected. 
file.ts(1,11): Error TS1005: ';' expected.
```
[Apparently not!](http://blogs.msdn.com/b/msbuild/archive/2006/11/03/msbuild-visual-studio-aware-error-messages-and-message-formats.aspx)