Implementation of S3 method for mtable class

[mtable class](http://www.inside-r.org/packages/cran/memisc/docs/mtable)
First I implemented without multi-line cells support, but in second commit added that. What do you think is better?
**Basic examples**
```
require(memisc)
lm0 <- lm(sr ~ pop15 + pop75,              data = LifeCycleSavings)
lm1 <- lm(sr ~                 dpi + ddpi, data = LifeCycleSavings)
lm2 <- lm(sr ~ pop15 + pop75 + dpi + ddpi, data = LifeCycleSavings)

mtable123 <- mtable("Model 1"=lm0,"Model 2"=lm1,"Model 3"=lm2,
                    summary.stats=c("sigma","R-squared","F","p","N"))
pander(mtable123)
berkeley <- aggregate(Table(Admit,Freq)~.,data=UCBAdmissions)

berk0 <- glm(cbind(Admitted,Rejected)~1,data=berkeley,family="binomial")
berk1 <- glm(cbind(Admitted,Rejected)~Gender,data=berkeley,family="binomial")
berk2 <- glm(cbind(Admitted,Rejected)~Gender+Dept,data=berkeley,family="binomial")

m1 <- mtable(berk0,summary.stats=c("Deviance","N"))
pander(m1)
m2 <- mtable(berk0,drop=FALSE,summary.stats=c("Deviance","N"))
pander(m2)
m3 <- mtable(berk1,summary.stats=c("Deviance","N"))
pander(m3)
m4 <- mtable(berk1,drop=FALSE,summary.stats=c("Deviance","N"))
pander(m4)
```