load XSL only once per XSLDocument

See #172, release log:

• 9ffcc70 by @yegor256: #172 pre-calculate transformer
• ec692e1 by @yegor256: #172 forSaxon
• 56e2a2d by @yegor256: #172 static TFACTORY
• 33d8b82 by @yegor256: #172 transformer()

Released by Rultor 2.0-SNAPSHOT, see build log