load extensions using the C API instead of SQL

[tajtiattila]

This fixes the problem of loading ICU, which needs to replace existing
functions. Replacing existing functions are prohibited when a VM
is running.

I found this problem on Windows x64, debugged by extending icu.c such that it returns a text message.

Another benefit of this change is that the SQL load_extension() function always returns NULL, but
now it is possible to examine the actual return value.

[coveralls]

Coverage increased (+0.23%) to 84.01% when pulling 0486dee on tajtiattila:master into a141177 on mattn:master.

[tajtiattila]

Note that the use of C.sqlite3_load_extension() might not be absolutely necessary: I haven't fully checked, but the SELECT load_extension... might still work without using a prepared statement
(i.e. use conn.Exec directly). Let me know if this should be checked.

I've verified that the calls to C.sqlite3_enable_load_extension() are still necessary. This observation corresponds to the sqlite documentation.

[mattn]

Thank you