Add support to ToLower()

This makes the following kind of query valid:

```
from p in DB.Table<Person> ()
where p.Name.ToLower ().Contains (searchName.ToLower ())
select p
```
Changing readme to remove old guidance for Win8 app section and pointing to SQLite.org as the source of truth for the sqlite binary in this case.
added support for calling sqlite3_enable_load_extension
Merge pull request #174 from fmcypriano/patch-1

Add support to ToLower()
Merge pull request #178 from ericsink/master

added support for sqlite3_enable_load_extension
error checking for ExecuteScalar
Merge pull request #173 from SeanCatalyst/master

Fix for WinRT compilation
Merge pull request #179 from ericsink/master

Another little patch
Merge pull request #2 from praeclarum/master

Merge changes