Add element_text with ability to add theme to axis.title.x

First pass at issue Label rotation #93. All keywords except lineheight seem to be working.

    ggplot(df, aes('factor(approved_type_of_work)')) +\
           geom_bar() +\
               theme(axis_title_x=element_text(angle=45,
                                               hjust=1,
                                               family="serif",
                                               face="bold.italic",
                                               color="green",
                                               size=20,
                                               vjust=1,
                                               lineheight=1.2))
![element_text](https://f.cloud.github.com/assets/2405373/2241985/98af4390-9cee-11e3-97dc-6d6434feb687.png)