Add support for property types:  XElement, DateTimeOffset, TimeSpan, multiple primary keys, NOT operator

Add support for multiple primary keys
Fix string not stored in i18n format
Implements NOT operator (patch from https://github.com/praeclarum/sqlite-net/issues/158)
Checked Android and Ios in the PCL targets

Notes: 
- some non functional changes were made by Visual Studio 2013.
- VS2013 PCL does not support WP7.1, so it can not be checked in the PCL profile. WP8 has been used instead.
- all tests passed