Update README.markdown

Updated supporting databases content in the overview based on wiki
Merge pull request #158 from gadalaavinash/patch-1

Update README.markdown
Updated merged npgsql library to a version without objectdispose bug
Merge pull request #159 from jenrom/master

Updated merged npgsql library to a version without objectdispose bug
Updated nuget search link

To point to correct search path
Merge pull request #164 from AkosLukacs/patch-1

Updated nuget search link