Implement base leveling system

[dlrocker]

This depends on #17

Implement the base design of the leveling system. This base design should only include:

• Mechanism to level up your pokemon
• Updating the level of the pokemon in the database for the user
• Any other aspects (such as "trading" pokemon in to level up another pokemon) based on design

This would not include:

• selecting moves a pokemon knows
• evolving a pokemon