Solarized Theme

Solarized theme as mentioned [here](https://github.com/benweet/stackedit/issues/451).

![stackedit-solarized-dark](https://cloud.githubusercontent.com/assets/3688847/4210749/5bd403b0-387b-11e4-8e00-e848a48a19c4.png)
![stackedit-solarized-dark-settings](https://cloud.githubusercontent.com/assets/3688847/4210751/5bdfb656-387b-11e4-8f9d-0e04b2ef06f4.png)
![stackedit-solarized-light](https://cloud.githubusercontent.com/assets/3688847/4210750/5bda350a-387b-11e4-9e3f-9fd437d75e9a.png)
![stackedit-solarized-light-dialog](https://cloud.githubusercontent.com/assets/3688847/4210752/5be454ae-387b-11e4-91b9-a6cd06664a61.png)