Modify layout styles for Material Design

Made layout changes to match the current [Material guidelines for Pickers](http://www.google.com/design/spec/components/pickers.html).

Android Lollipop has native `DatePicker`s and `TimePicker`s that look like the following:

<img src="https://cloud.githubusercontent.com/assets/375956/5152887/4274ed1e-71d6-11e4-88ca-0e9588a3bae3.png" align="left" height="350" width="200" >

<img src="https://cloud.githubusercontent.com/assets/375956/5152900/e34fba0c-71d6-11e4-9eb5-7ca9205aec9a.png" height="350" width="200" >

These new pickers are not ported to pre-Android 5.0, even with AppCompat, so I followed the guidelines as close as possible to create a similar look for this library:

<img src="https://cloud.githubusercontent.com/assets/375956/5152908/825f94a0-71d7-11e4-8213-f95a1a9af78f.png" align="left" height="350" width="200" >

<img src="https://cloud.githubusercontent.com/assets/375956/5152912/ab7e3814-71d7-11e4-87fa-346ce5e3783a.png" height="350" width="200" >

<img src="https://cloud.githubusercontent.com/assets/375956/5152921/6ee5945a-71d8-11e4-87f7-a49bf8e559e3.png" height="200" width="350" >

<img src="https://cloud.githubusercontent.com/assets/375956/5152922/903774ac-71d8-11e4-8c03-3103b2378455.png" height="200" width="350" >

while still retaining the original color theme used in this repo (if anyone needs to use custom colors, please see https://github.com/flavienlaurent/datetimepicker/issues/65#issuecomment-58580121)

Changes made are mostly to layout or style xml files. Commits are self-contained and easy to review. 

Tested on Nexus 7, 5 & 4.