Fixed a bug in MimeParser.StepHeaders()

If we don't advance any bytes this loop, don't set midline to true.

Fixes a case where the '\r' of a blank line that terminates the headers
is the last byte of the previous read(). If we set midline to true, then
what happens is that we assume that the blank line was part of the headers
and never stop scanning the headers until we reach the end of the file.

Thanks to Hugo Hallqvist for finding & reporting this bug.
minor formatting fix
Made X509-related types public

This is a necessary step to make possible to implement different
IX509CertificateDatabase providers and remove the dependency on
Mono.Data.Sqlite.
Removed dependency on Mono.Data.Sqlite

SqliteCertificateDatabase now loads Mono.Data.Sqlite using reflection and
doesn't depend on it explicitly. A new property "IsAvailable" can be used
to determine if SQLite support is available at runtime.

DefaultSecureMimeContext now checks for SQLite support at runtime and
throws an exception if not available. This can be replaced by a better
behaviour in the future (using a different IX509CertificateDatabase) or
left as is.
Merge pull request #48 from fogzot/master

Removal of Mono.Data.Sqlite dependency
Modified DbExtensions.CreateParameterWithValue() to be DbExtensions.AddParameterWithValue()
Don't use reflection for Mono.Data.Sqlite for Xamarin mobile platforms
Added a new .ctor to DefaultSecureMimeContext and made SqliteCertificateDatabase public
Documented most of the newly exposed X509 certificate database APIs
Finished documenting all of the new database APIs
Added a work-around for mail clients that mistakenly quote rfc2184 encoded parameter values

Fixes issue #49
Added more runtime checks to make sure Mono.Data.Sqlite will work
Bumped version to 0.32.0.0
Removed sqlite3.dll and Mono.Data.Sqlite from MimeKit.nuspec
Updated unit tests
Documented how to register your own CryptographyContexts
Allow parsing of message with 0 byte body.
Merge pull request #51 from hultqvist/master

Allow parsing of message with 0 byte body.
Cleaned up previous pull request a bit and added some commentary
Fixed compile warning by commenting out one-pass stuff that we don't use