Initial DDEX support : new pr

A new pull request.
Initial DDEX support code is inherited from #67 started by wwindcloud.
Note: I have manually rebased previous code. So, it may contain some problems...

DDEX tips:
- DDEX support will work on VS2010/VS2012/VS2013 Pro (VS2013 needs [Visual Studio 2013 Update 2](http://www.microsoft.com/en-us/download/details.aspx?id=42666) or later) (*Edited, 3 Jun*)
- Specify all of Host, Username, Password and **Database** properties in DDEX connection dialog.
- Capture your ConnectionString for later use. For example,
```
PORT=5432;TIMEOUT=15;POOLING=True;MINPOOLSIZE=1;MAXPOOLSIZE=20;COMMANDTIMEOUT=20;COMPATIBLE=2.1.0.0;HOST=127.0.0.1;USER ID=npgsql_tests;PASSWORD=npgsql_tests;DATABASE=npgsql_tests
```

Built tips:
- Open *Npgsql2012.sln*.
- Configuration: *Debug-net45*, *Release-net45* or such.
- Built the entire solution.
- Install the generated vsix. For example, NpgsqlDdexProvider\bin\Debug-net45\NpgsqlDdexProvider.vsix
- gacutil /i  Npgsql.dll

(Edit 22 Jun, 2014)
You'll need gacutil for .NET runtime 4.0.
It will be found from one of following folders:
- C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin\NETFX 4.0 Tools\gacutil.exe
- C:\Program Files (x86)\Microsoft SDKs\Windows\v8.0A\bin\NETFX 4.0 Tools\gacutil.exe
- C:\Program Files (x86)\Microsoft SDKs\Windows\v8.1A\bin\NETFX 4.5.1 Tools\gacutil.exe