Fix misplaced input segmented buttons in FF and IE and support for dropdowns positioning

The float left used to collapse borders causes the IE and FF troubles, same can be achieved using the negative margin that already exists.

btn-group can be used inside input-groups as well in order to place dropdowns

Tested in FF, Safari, Chrome, IE 10, 9, 8 in all media cases and with all possible combination, with multiple drop downs, and buttons mixed.

**Before**
![before](https://f.cloud.github.com/assets/38347/713145/e85df74c-debc-11e2-8f20-c52f567f1663.png)

**After**
![after](https://f.cloud.github.com/assets/38347/713147/ee9e3e5a-debc-11e2-91c7-377d26aa6fa8.png)

**Extra Mixed tests**
![captura de pantalla 2013-06-27 a la s 02 06 14](https://f.cloud.github.com/assets/38347/713166/796ec950-debd-11e2-91e3-487dbf9d035b.png)